Hybrid vehicles

ABSTRACT

A hybrid vehicle comprises an internal combustion engine, a traction motor, a starter motor, and a battery bank, all controlled by a microprocessor in accordance with the vehicle&#39;s instantaneous torque demands so that the engine is run only under conditions of high efficiency, typically only when the load is at least equal to 30% of the engine&#39;s maximum torque output. In some embodiments, a turbocharger may be provided, activated only when the load exceeds the engine&#39;s maximum torque output for an extended period; a two-speed transmission may further be provided, to further broaden the vehicle&#39;s load range. A hybrid brake system provides regenerative braking, with mechanical braking available in the event the battery bank is fully charged, in emergencies, or at rest; a control mechanism is provided to control the brake system to provide linear brake feel under varying circumstances.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 09/264,817, filedMar. 9, 1999, now U.S. Pat. No. 6,209,672, issued Apr. 3, 2001, which inturn claims priority from provisional application Ser. No. 60/100,095,filed Sep. 14, 1998, and is also a continuation-in-part of Ser. No.09/392,743, filed Sep. 9, 1999, which in turn claims priority fromprovisional application Ser. No. 60/122,296, filed Mar. 1, 1999.

FIELD OF THE INVENTION

This application relates to improvements in hybrid vehicles, that is,vehicles in which both an internal combustion engine and one or moreelectric motors are provided to supply torque to the driving wheels ofthe vehicle. More particularly, this invention relates to a hybridelectric vehicle that is fully competitive with presently conventionalvehicles as regards performance, operating convenience, and cost, whileachieving substantially improved fuel economy and reduced pollutantemissions.

DISCUSSION OF THE PRIOR ART

For many years great attention has been given to the problem ofreduction of fuel consumption of automobiles and other highway vehicles.Concomitantly very substantial attention has been paid to reduction ofpollutants emitted by automobiles and other vehicles. To a degree,efforts to solve these problems conflict with one another. For example,increased thermodynamic efficiency and thus reduced fuel consumption canbe realized if an engine is operated at higher temperatures. Thus therehas been substantial interest in engines built of ceramic materialswithstanding higher combustion temperatures than those now in use.However, higher combustion temperatures in gasoline-fueled engines leadto increase in certain undesirable pollutants, typically NO_(x).

Another possibility for reducing emissions is to burn mixtures ofgasoline and ethanol (“gasohol”), or straight ethanol. However, to dateethanol has not become economically competitive with gasoline, andconsumers have not accepted ethanol to any great degree. Moreover, tomake an alternate fuel such as ethanol available to the extent necessaryto achieve appreciable improvements in nationwide air quality and fuelconservation would require immense costs for infrastructureimprovements; not only the entire nation's motor fuel production anddelivery system, but also the vehicle manufacture, distribution, andrepair system, would have to be extensively revised or substantiallyduplicated.

One proposal for reducing pollution in cities is to limit the use ofvehicles powered by internal combustion engines and instead employelectric vehicles powered by rechargeable batteries. To date, all such“straight electric” cars have had very limited range, typically no morethan 150 miles, have insufficient power for acceleration and hillclimbing except when the batteries are substantially fully charged, andrequire substantial time for battery recharging. Thus, while there aremany circumstances in which the limited range and extended rechargingtime of the batteries would not be an inconvenience, such cars are notsuitable for all the travel requirements of most individuals.Accordingly, an electric car would have to be an additional vehicle formost users, posing a substantial economic deterrent. Moreover, it willbe appreciated that in the United States most electricity is generatedin coal-fired power plants, so that using electric vehicles merely movesthe source of the pollution, but does not eliminate it. Furthermore,comparing the respective net costs per mile of driving, electricvehicles are not competitive with ethanol-fueled vehicles, much lesswith conventional gasoline-fueled vehicles. see, generally, Simanaitis,“Electric Vehicles”, Road & Track, May 1992, pp. 126-136; Reynolds, “ACPropulsion CRX”, Road & Track, October 1992, pp. 126-129.

Brooks et al U.S. Pat. No. 5,492,192 shows such an electric vehicle; theinvention appears to be directed to incorporation of antilock brakingand traction control technologies into an otherwise conventionalelectric vehicle.

Much attention has also been paid over the years to development ofelectric vehicles including internal combustion engines poweringgenerators, thus eliminating the defect of limited range exhibited bysimple electric vehicles. The simplest such vehicles operate on the samegeneral principle as diesel-electric locomotives used by most railroads.In such systems, an internal combustion engine drives a generatorproviding electric power to traction motors connected directly to thewheels of the vehicle. This system has the advantage that no variablegear ratio transmission is required between the engine and the wheels ofthe vehicle.

More particularly, an internal combustion engine produces zero torque atzero engine speed (RPM) and reaches its torque peak somewhere in themiddle of its operating range. Accordingly, all vehicles driven directlyby an internal combustion engine (other than certain single-speedvehicles using friction or centrifugal clutches, and not useful fornormal driving) require a variable-ratio transmission between the engineand the wheels, so that the engine's torque can be matched to the roadspeeds and loads encountered. Further, some sort of clutch must beprovided so that the engine can be mechanically decoupled from thewheels, allowing the vehicle to stop while the engine is still running,and to allow some slippage of the engine with respect to the drive trainwhile starting from a stop. It would not be practical to provide adiesel locomotive, for example, with a multiple speed transmission, or aclutch. Accordingly, the additional complexity of the generator andelectric traction motors is accepted. Electric traction motors producefull torque at zero RPM and thus can be connected directly to thewheels; when it is desired that the train should accelerate, the dieselengine is simply throttled to increase the generator output and thetrain begins to move.

The same drive system may be employed in a smaller vehicle such as anautomobile or truck, but has several distinct disadvantages in thisapplication. In particular, and as discussed in detail below inconnection with FIGS. 1 and 2, it is well known that a gasoline or otherinternal combustion engine is most efficient when producing near itsmaximum output torque. Typically, the number of diesel locomotives on atrain is selected in accordance with the total tonnage to be moved andthe grades to be overcome, so that all the locomotives can be operatedat nearly full torque production. Moreover, such locomotives tend to berun at steady speeds for long periods of time. Reasonably efficient fueluse is thus achieved. However, such a direct drive vehicle would notachieve good fuel efficiency in typical automotive use, involving manyshort trips, frequent stops in traffic, extended low-speed operation andthe like.

So-called “series hybrid” electric vehicles have been proposed forautomotive use, wherein batteries are used as energy storage devices, sothat an internal combustion engine provided to power a generator can beoperated in its most fuel-efficient output power range while stillallowing the electric traction motor(s) powering the vehicle to beoperated as required. Thus the engine may be loaded by supplying torqueto a generator charging the batteries while supplying electrical powerto the traction motor(s) as required, so as to operate efficiently. Thissystem overcomes the limitations of electric vehicles noted above withrespect to limited range and long recharge times. Thus, as compared to aconventional vehicle, wherein the internal combustion engine deliverstorque directly to the wheels, in a series hybrid electric vehicle,torque is delivered from the engine to the wheels via a seriallyconnected generator used as a battery charger, the battery, and thetraction motor. However, energy transfer between those componentsconsumes at least approximately 25% of engine power. Further, suchcomponents add substantially to the cost and weight of the vehicle; inparticular, an electric motor capable of providing sufficient torque tomeet all expected demand, e.g., to allow reasonable performance underacceleration, during hill-climbing and the like, is rather heavy andexpensive. Thus, series hybrid vehicles have not been immediatelysuccessful.

A more promising “parallel hybrid” approach is shown in U.S. Pat. Nos.3,566,717 and 3,732,751 to Berman et al. In Berman et al an internalcombustion engine and an electric motor are matched through a complexgear train so that both can provide torque directly to the wheels, thevehicle being operated in several different modes. Where the output ofthe internal combustion engine is more than necessary to drive thevehicle (“first mode operation”) the engine is run at constant speed andexcess power is converted by a first motor/generator (“speeder”) toelectrical energy for storage in a battery. In “second mode operation”,the internal combustion engine drives the wheels directly, and isthrottled. When more power is needed than the engine can provide, asecond motor/generator or “torquer” provides additional torque asneeded.

Berman et al thus show two separate electric motor/generators separatelypowered by the internal combustion engine; the “speeder” charges thebatteries, while the “torquer” propels the vehicle forward in traffic.This arrangement is a source of additional complexity, cost anddifficulty, as two separate modes of engine control are required.Moreover, the operator. must control the transition between the severalmodes of operation. Such a complex vehicle is unsuited for theautomotive market. Automobiles intended for mass production can be nomore complicated to operate than conventional vehicles, and must beessentially “foolproof”, that is, resistant to damage that might becaused by operator error. Further, the gear train shown by Berman et alappears to be quite complex and difficult to manufacture economically.Berman et al also indicate that one or even two variable-speedtransmissions may be required; see, e.g., col. 3, lines 19-22 and 36-38of U.S. Pat. No. 3,566,717, and col. 2, lines 53-55 of U.S. Pat. No.3,732,751.

Lynch et al U.S. Pat. No. 4,165,795 also shows an early parallel hybriddrive. Lynch argues that maximum fuel efficiency can be realized when arelatively small internal combustion engine is provided, such that whenthe engine is operated at an efficient speed, it produces approximatelythe average power required over a typical mission. The example given isof an engine producing 25 hp maximum and 17 hp at its most efficientspeed, about 2500 rpm. This is to be combined with an electricmotor-generator of about 30 peak hp. This vehicle requires avariable-ratio transmission to achieve reasonable performance. Itappears that the engine is to be run continuously, at a steady speed,with additional torque provided by the motor when needed and excesstorque produced by the engine being used to charge the batteries. In afirst embodiment, torque provided by the motor is transmitted to thedrive wheels through the engine, while in a second embodiment theirrespective positions are reversed.

Nishida U.S. Pat. No. 5,117,931 shows a parallel hybrid vehicle wheretorque from an electric motor may be combined with torque from aninternal combustion engine in a “torque transmission unit” comprisingpaired bevel gears and means for controlling the relative rates ofrotation of the motor and engine, so that the motor can be used to startthe engine, absorb excess torque from the engine (by charging abattery), or provide additional propulsive torque. A variable-speedtransmission is coupled between the torque transmission unit and thepropelling wheels. Both the torque transmission unit and thevariable-speed transmission are complex, heavy, and expensivecomponents, the use of which would preferably be avoided.

Helling U.S. Pat. No. 3,923,115 also shows a hybrid vehicle having atorque transmission unit for combining torque from an electric motor andan internal combustion engine. However, in Helling the relative rates ofrotation of the motor and engine input shafts are fixed; a flywheel isprovided to store excess mechanical energy as well as a battery to storeexcess electrical energy. Albright, Jr. et al U.S. Pat. No. 4,588,040shows another hybrid drive scheme using a flywheel in addition tobatteries to store excess energy; various complicated mechanicalconnections are provided between the various components. Capacitors havealso been proposed for energy storage; see Bates et al U.S. Pat. No.5,318,142.

Fjällström U.S. Pat. No. 5,120,282 shows a parallel hybrid drive trainwherein torque from two electric motors is combined with torque producedby an internal combustion engine; the combination is performed by acomplex arrangement of paired planetary gearsets, and unspecifiedcontrol means are alleged to be able to allow variation of road speedwithout a variable-ratio transmission.

Hunt U.S. Pat. Nos. 4,405,029 and 4,470,476 also disclose parallelhybrids requiring complex gearing arrangements, including multiple speedtransmissions. More specifically, the Hunt patents disclose severalembodiments of parallel hybrid vehicles. Hunt indicates (see col. 4,lines 6-20 of the '476 patent) that an electric motor may drive thevehicle at low speeds up to 20 mph, and an internal combustion engineused for speeds above 20 mph, while “in certain speed ranges, such asfrom 15-30 mph, both power sources may be energized . . . .Additionally, both power sources could be utilized under heavy loadconditions.” Hunt also indicates that “the vehicle could be providedwith an automatic changeover device which automatically shifts from theelectrical power source to the internal combustion power source,depending on the speed of the vehicle” (col. 4, lines 12-16).

However, the Hunt vehicle does not meet the objects of the presentinvention, as discussed in detail below. Hunt's vehicle in eachembodiment requires a conventional manual or automatic transmission. Seecol. 2, lines 6-7. Moreover, the internal combustion engine is connectedto the transfer case (wherein torque from the internal combustion engineand electric motor is combined) by a “fluid coupling or torque converterof conventional construction”. Col. 2, lines 16-17. Such transmissionsand fluid couplings or torque converters are very inefficient, areheavy, bulky, and costly, and are to be eliminated according to oneobject of the present invention, again as discussed in detail below.

Furthermore, the primary means of battery charging disclosed by Huntinvolves a further undesirable complexity, namely a turbine driving theelectric motor in generator configuration. The turbine is fueled bywaste heat from the internal combustion engine. See col. 3, lines 10-60.Hunt's internal combustion engine is also fitted with an alternator, foradditional battery charging capability, adding yet further complexity.Thus it is clear that Hunt fails to teach a hybrid vehicle meeting theobjects of the present invention—that is, a hybrid vehicle competitivewith conventional vehicles with respect to performance, cost andcomplexity, while achieving substantially improved fuel efficiency.

Kawakatsu U.S. Pat. Nos. 4,305,254 and 4,407,132 show a parallel hybridinvolving a single internal combustion engine coupled to the drivewheels through a conventional variable-ratio transmission, an electricmotor, and an alternator, to allow efficient use of the internalcombustion engine. As in the Hunt disclosure, the engine is intended tobe operated in a relatively efficient range of engine speeds; when itproduces more torque than is needed to propel the vehicle, the excess isused to charge the batteries; where the engine provides insufficienttorque, the motor is energized as well.

A further Kawakatsu U.S. Pat. No. 4,335,429, shows a hybrid vehicle, inthis case comprising an internal combustion engine and twomotor/generator units. A first larger motor/generator, powered by abattery, is used to provide additional torque when that provided by theengine is insufficient; the larger motor-generator also converts excesstorque provided by the engine into electrical energy, to be stored bythe battery, and is used in a regenerative braking mode. The secondsmaller motor/generator is similarly used to provide additional torqueand additional regenerative braking as needed.

More particularly, the latter Kawakatsu patent asserts that a singleelectric motor sized to provide sufficient torque to propel the vehiclewould not be capable of providing sufficient regenerative braking force;see col. 1, line 50—col. 2 line 8. Accordingly, Kawakatsu provides twoseparate motor/generators, as noted; a separate engine starting motor isalso provided. See col. 6, lines 22-23. In the embodiment shown, thelarger motor/generator is connected to the wheel drive shaft, while theengine and the smaller motor/generator are connected to the wheelsthrough a complex mechanism comprising three separately-controllableclutches. See col. 5, lines 50-62.

Numerous patents disclose hybrid vehicle drives tending to fall into oneor more of the categories discussed above. A number of patents disclosesystems wherein an operator is required to select between electric andinternal combustion operation; for example, an electric motor isprovided for operation inside buildings where exhaust fumes would bedangerous, and an internal combustion engine provided for operationoutdoors. It is also known to propose a hybrid vehicle comprising anelectric motor for use at low speeds, and an internal combustion enginefor use at higher speed. The art also suggests using both when maximumtorque is required. In several cases the electric motor drives one setof wheels and the internal combustion engine drives a different set. Seegenerally Shea (U.S. Pat. No. 4,180,138); Fields et al (U.S. Pat. No.4,351,405); Kenyon (U.S. Pat. No. 4,438,342); Krohling (U.S. Pat. No.4,593,779); and Ellers (U.S. Pat. No. 4,923,025).

Many of these patents show hybrid vehicle drives wherein a variablespeed transmission is required, as do numerous additional references. Atransmission as noted above is typically required where the internalcombustion engine and/or the electric motor are not capable of supplyingsufficient torque at low speeds. See Rosen (U.S. Pat. No. 3,791,473);Rosen (U.S. Pat. No. 4,269,280); Fiala (U.S. Pat. No. 4,400,997); and Wuet al (U.S. Pat. No. 4,697,660). Kinoshita (U.S. Pat. No. 3,970,163)shows a vehicle of this general type wherein a gas turbine engine iscoupled to the road wheels through a three-speed transmission; anelectric motor is provided to supply additional torque at low speeds.

For further examples of series hybrid vehicles generally as discussedabove, see Bray (U.S. Pat. No. 4,095,664); Cummings (U.S. Pat. No.4,148,192); Monaco et al (U.S. Pat. No. 4,306,156); Park (U.S. Pat. No.4,313,080); McCarthy (U.S. Pat. No. 4,354,144); Heidemeyer (U.S. Pat.No. 4,533,011); Kawamura (U.S. Pat. No. 4,951,769); and Suzuki et al(U.S. Pat. No. 5,053,632). Various of these address specific problemsarising in the manufacture or use of hybrid vehicles, or specificalleged design improvements. For example, Park addresses certainspecifics of battery charging and discharge characteristics, whileMcCarthy shows a complex drive system involving an internal combustionengine driving two electric motors; the torque generated by the latteris combined in a complex differential providing continuously variablegear ratios. Heidemeyer shows connecting an internal combustion engineto an electric motor by a first friction clutch, and connecting themotor to a transmission by a second friction clutch.

Other patents of general relevance to this subject matter include Toy(U.S. Pat. No. 3,525,874), showing a series hybrid using a gas turbineas internal combustion engine; Yardney (U.S. Pat. No. 3,650,345),showing use of a compressed-air or similar mechanical starter for theinternal combustion engine of a series hybrid, such that batteries oflimited current capacity could be used; and Nakamura (U.S. Pat. No.3,837,419), addressing improvements in thyristor battery-charging andmotor drive circuitry. Somewhat further afield but of general interestare the disclosures of Deane (U.S. Pat. No. 3,874,472); Horwinski (U.S.Pat. No. 4,042,056); Yang (U.S. Pat. No. 4,562,894); Keedy (U.S. Pat.No. 4,611,466); and Lexen (U.S. Pat. No. 4,815,334); Mori (U.S. Pat. No.3,623,568); Grady, Jr. (U.S. Pat. No. 3,454,122); Papst (U.S. Pat. No.3,211,249); Nims et al (U.S. Pat. No. 2,666,492); and Matsukata (U.S.Pat. No. 3,502,165). Additional references showing parallel hybridvehicle drive systems include Froelich (U.S. Pat. No. 1,824,014) andReinbeck (U.S. Pat. No. 3,888,325). U.S. Pat. No. 4,578,955 to Medinashows a hybrid system wherein a gas turbine is used to drive a generatoras needed to charge batteries. Of particular interest to certain aspectsof the present invention is that Medina discloses that the battery packshould have a voltage in the range of 144, 168 or 216 volts and thegenerator should deliver current in the range of 400 to 500 amperes.Those of skill in the art will recognize that these high currentsinvolve substantial resistance heating losses, and additionally requirethat all electrical connections be made by positive mechanical meanssuch as bolts and nuts, or by welding. More specifically, for reasons ofsafety and in accordance with industry practice, currents in excess ofabout 50 amperes cannot be carried by the conventional plug-inconnectors preferred for reasons of convenience and economy, but must becarried by much heavier, more expensive and less convenient fixedconnectors (as used on conventional starter and battery cableconnections). Accordingly, it would be desirable to operate the electricmotor of a hybrid vehicle at lower currents.

U.S. Pat. No. 5,765,656 to Weaver also shows a series hybrid wherein agas turbine is used as the internal combustion engine; hydrogen is thepreferred fuel.

U.S. Pat. No. 4,439,989 to Yamakawa shows a system wherein two differentinternal combustion engines are provided, so that only one need be runwhen the load is low. This arrangement would be complex and expensive tomanufacture.

Detailed discussion of various aspects of hybrid vehicle drives may befound in Kalberlah, “Electric Hybrid Drive Systems for Passenger Carsand Taxis”, SAE Paper No. 910247 (1991). Kalberlah first compares“straight” electric, series hybrid, and parallel hybrid drive trains,and concludes that parallel hybrids are preferable, at least whenintended for general use (that is, straight electric vehicles may beuseful under certain narrow conditions of low-speed, limited range urbandriving). Kalberlah then compares various forms of parallel hybrids,with respect to his FIG. 4, and concludes that the most practicalarrangement is one in which an internal combustion engine drives a firstpair of wheels, and an electric motor the second; more particularly,Kalberlah indicates that mechanical combination of the torque from aninternal combustion engine and an electric motor is impractical.

Gardner U.S. Pat. Nos. 5,301,764 and 5,346,031 follow Kalberlah'steachings, in that Gardner shows separately driving at least two pairsof wheels; one pair is driven by a first electric motor, and the secondby a second electric motor or alternatively by a small internalcombustion engine. Three different clutches are provided to allowvarious sources of drive torque to be connected to the wheels, and to agenerator, depending on the vehicle's operation mode. The internalcombustion engine is run continuously, and provides the driving torquewhen the vehicle is in a cruise mode; at other times it is used tocharge the batteries powering the electric motors.

Bullock, “The Technological Constraints of Mass, Volume, Dynamic PowerRange and Energy Capacity on the Viability of Hybrid and ElectricVehicles”, SAE Paper No. 891659 (1989) provides a detailed theoreticalanalysis of electric vehicles in terms of the loads thereon, and acareful analysis of the various battery types then available. Bullockconcludes that a vehicle having two electric motors of differingcharacteristics, driving the wheels through a variable-speedtransmission, would be optimal for automotive use; see the discussion ofFIG. 8. Bullock also suggests the use of an internal combustion engineto drive battery charging, but does not address combining the engine'storque with that from the motors; see pp. 24-25.

Further related papers are collected in Electric and Hybrid VehicleTechnology, volume SP-915, published by SAE in February 1992. See alsoWouk, “Hybrids: Then and Now”; Bates, “On the road with a Ford HEV”, andKing et al, “Transit Bus takes the Hybrid Route”, all in IEEE Spectrum,Vol. 32, 7, (July 1995).

Urban et al U.S. Pat. No. 5,667,029 shows two embodiments of parallelhybrids; a first embodiment is shown in FIGS. 1-9 and 11, and a secondin FIGS. 12-17. Both embodiments have numerous common features,including similar operating modes. Referring to the first embodiment, aninternal combustion engine provides torque to the road wheels or to agenerator; two electric motors can provide torque to the road wheels, orcharge batteries during regenerative braking. Torque from the engine andmotors is combined at the input shaft to a variable-ratio transmission.Overrunning clutches are provided, e.g., to allow the engine's torque tobe applied to the road wheels without also rotating the motors.

As indicated at col. 6, lines 25-54, certain transitions between variousoperating modes are made automatically, responsive to the position ofthe accelerator pedal; for example, if the operator does not depress thepedal beyond a given point, only the internal combustion engine isemployed to propel the vehicle; if the operator depresses the pedal morefully, the electric motors are also energized. Other changes in theoperational mode must be made by the operator directly; for example, thevehicle may be operated as a “straight electric” vehicle, e.g. for shortduration trips, by the operator's making an appropriate control action.See col. 7, lines 49-56.

The Urban et al design appears to suffer from a number of significantdefects. First, the internal combustion engine is stated to provide alltorque needed to accelerate the vehicle to cruising speed under normalcircumstances (see col. 5, lines 3-10), and also to propel the vehicleduring cruising (see col. 6, lines 48-54). The electric motors are to beused only during rapid acceleration and hill-climbing; col. 5, lines10-13. A 20 horsepower engine, operated through a continuouslyvariable-ratio transmission and a torque converter, is stated to beadequate for this purpose. Such components are clearly complex andexpensive; further, torque converters are notoriously inefficient.Moreover, using the internal combustion engine as the sole source ofpower for low-speed running would require it to be run at low speeds,e.g., at traffic lights, which is very inefficient and highly polluting.(Various additional references suggest that excess torque can be used tocharge batteries; if this were incorporated in the Urban system, theengine might be run at a reasonably efficient output level while thevehicle was stationary, but this would lead to high levels of noise andvibration. In any event Urban does not appear to consider thispossibility.)

On the other hand, Urban does suggest that the vehicle can be operatedas a “straight electric” under low-speed conditions, but this requiresthe operator to provide an explicit control input; this complexity isunacceptable in a vehicle intended to be sold in quantity, as would berequired in order to reach Urban's stated goals of reduction ofatmospheric pollution and reduced energy consumption. As noted, hybridvehicle operation must be essentially “foolproof”, or “transparent” tothe user, to have any chance of commercial success.

Urban's second embodiment is mechanically simpler, employing but asingle “dynamotor”, through which torque is transmitted from the engineto the variable-ratio transmission, but suffers from the sameoperational deficiencies.

A second Urban et al U.S. Pat. No. 5,704,440, is directed to the methodof operation of the vehicle of the '029 patent and suffers the sameinadequacies.

Various articles describe several generations of Toyota Motor Companyhybrid vehicles, believed to correspond to that available commerciallyas the “Prius”. See, for example, Yamaguchi, “Toyota readiesgasoline/electric hybrid system”, Automotive Engineering, July 1997, pp.55-58; Wilson, “Not Electric, Not Gasoline, But Both”, Autoweek, Jun. 2,1997, pp. 17-18; Bulgin, “The Future Works, Quietly”, Autoweek Feb. 23,1998, pp. 12 and 13; and “Toyota Electric and Hybrid Vehicles”, a Toyotabrochure. A more detailed discussion of the Toyota vehicle's powertrainis found in Nagasaka et al, “Development of the Hybrid/Battery ECU forthe Toyota Hybrid System”, SAE paper 981122 (1998), pp. 19-27.

According to the Wilson article, Toyota describes this vehicle as a“series-parallel hybrid”; regardless of the label applied, itspowertrain appears to be similar to that of the Berman patents describedabove, that is, torque from either or both of an internal combustionengine and an electric motor are controllably combined in a “power-splitmechanism” and transmitted to the drive wheels through a planetarygearset providing the functionality of a variable-ratio transmission.See the Nagasaka article at pp. 19-20.

Furutani U.S. Pat. No. 5,495,906 describes a vehicle having an internalcombustion engine driving a first set of wheels through a variable-ratiotransmission and an electric motor driving a second set of wheels. Theengine is apparently intended to be run continuously; at low speeds, itdrives a generator to charge batteries providing energy to the motor,and at higher speeds the engine or both engine and motor propel thevehicle. In some circumstances the transmission may not be required;compare, for example, col. 3, lines 4-8 with col. 5, lines 59-64.

U.S. Pat. No. 5,842,534 to Frank shows a “charge depletion” controlmethod for hybrid vehicles; in this scheme, the internal combustionengine is essentially used only when the state of the batteries is suchthat the vehicle cannot otherwise reach a recharging point. See col. 3,lines 50-55. In normal operation, the batteries are recharged from anexternal power source. Frank also discusses two-mode brake pedaloperation, wherein mechanical brakes are engaged in addition toregenerative braking when the pedal is depressed beyond a preset point.

U.S. Pat. No. 5,823,280 to Lateur et al shows a parallel hybrid whereinthe shafts of an internal combustion engine and first and secondelectric motors are all coaxial; the engine is connected to the firstmotor by a clutch, and the first motor to the second by a planetarygearset, allowing the speeds of the motors to be varied so as to operatethem in their most efficient range. See col. 4, line 57—col. 5, line 60.

U.S. Pat. No. 5,826,671 to Nakae et al shows a parallel hybrid whereintorque from an internal combustion engine is combined with that from amotor in a planetary gearset; a clutch is provided therebetween. Thespecific invention relates to sensing of engine warmup conditions, so asto limit emission of unburnt fuel and thus lower emissions.

U.S. Pat. No. 5,846,155 to Taniguchi et al shows a parallel hybridwherein torque from an internal combustion engine and a motor is againcombined in a planetary gearset; the specific improvement appears to bethe use of a continuously-variable transmission.

It will be appreciated by those of skill in the art that there aresignificant limitations inherent in the use of planetary gearsets as ameans for connecting different sources, e.g., an internal combustionengine and an electric motor, to the drive wheels of a vehicle, namely,that unless the planetary gearset is effectively locked (anathematic toits use as a continuously-variable transmission, e.g., in the Toyotavehicle) it is capable of additive combination of shaft speeds, but notof output torque. Hence, the principal advantage of the parallel hybriddrivetrain, additive combination of the output torque of both theelectric motor and the internal combustion engine, is only availablewhen the planetary gearset is locked. This fact is acknowledged byLateur, for example, at col. 6, line 27.

Additional disclosures of possible interest include U.S. Pat. No.5,845,731 to Buglione et al; this patent issued Dec. 8, 1998, andtherefore is not necessarily available as a reference against the claimsof the present application. The basic powertrain shown by Buglione et alincludes an internal combustion engine 12, coupled through a firstclutch 18 to a first electric motor 20, coupled to a second electricmotor 26 through a second clutch 24; the wheels are (apparently; seecol. 3, line 8) driven by the second motor 26. The overall hybridoperational scheme provided by Buglione et al is illustrated in FIG. 4.At low speeds one or both motors may be used to propel the vehicle, withthe engine off, idling, or running to drive one motor as a generator.During low-speed cruising the second motor propels the vehicle, whileduring high-speed cruising, the engine propels the vehicle. Whenacceleration is required at high speed, the engine and both motors maybe used to propel the vehicle. Buglione et al also indicates that avariable-ratio transmission may be unnecessary, col. 3, line 9, and thatthe first motor can be used to start the engine, col. 4, lines 8-15.

U.S. Pat. No. 5,586,613 to Ehsani, showing an “electrically peakinghybrid” vehicle is also of interest. Ehsani's vehicle is shown inseveral embodiments; in each, an engine is apparently to be runcontinuously, with excess torque used to charge the batteries, and oneor more motors used to provide additional propulsive torque when theengine's output torque is inadequate. A transmission is provided in someembodiments of the Ehsani vehicle.

An embodiment involving two motors is shown in FIG. 7, and can bemodified as discussed in the text at col. 9, lines 4-5. FIG. 7 itselfshows driving a first set of wheels by a first “electric machine”, i.e.,a motor capable of operation as a generator. This drive arrangement isindependent of a second drive arrangement, whereby a second set ofwheels is driven by an engine connected through a first clutch to asecond electric machine, connected to the second set of wheels by asecond clutch. Ehsani suggests at col. 9, lines 4-5 that the drive shaftotherwise coupled to the first electric machine could also be driven bythe engine. Although it is not made explicit that the first electricmachine is to be retained, this seems likely; otherwise, the modifiedFIG. 7 embodiment would be the same as Ehsani's FIG. 1, modified to haveall four wheels driven by a common driveshaft.

This application discloses a number of improvements over andenhancements to the hybrid vehicles disclosed in U.S. Pat. No. 5,343,970(the “'970 patent”), to one of the present inventors, which isincorporated herein by this reference. Where differences are notmentioned, it is to be understood that the specifics of the vehicledesign shown in the '970 patent are applicable to the vehicles shownherein as well. Discussion of the '970 patent herein is not to beconstrued to limit the scope of its claims.

Generally speaking, the '970 patent discloses hybrid vehicles wherein acontrollable torque transfer unit is provided capable of transferringtorque between an internal combustion engine, an electric motor, and thedrive wheels of the vehicle. The direction of torque transfer iscontrolled by a microprocessor responsive to the mode of operation ofthe vehicle, to provide highly efficient operation over a wide varietyof operating conditions, and while providing good performance. The flowof energy—either electrical energy stored in a substantial battery bank,or chemical energy stored as combustible fuel—is similarly controlled bythe microprocessor.

For example, according to the operating scheme of the hybrid vehicledisclosed in the '970 patent, in low-speed city driving, the electricmotor provides all torque needed responsive to energy flowing from thebattery. In high-speed highway driving, where the internal-combustionengine can be operated efficiently, it typically provides all torque;additional torque may be provided by the electric motor as needed foracceleration, hill-climbing, or passing. The electric motor is also usedto start the internal-combustion engine, and can be operated as agenerator by appropriate connection of its windings by a solid-state,microprocessor-controlled inverter. For example, when the state ofcharge of the battery bank is relatively depleted, e.g., after a lengthyperiod of battery-only operation in city traffic, the internalcombustion engine is started and drives the motor at between 50 and 100%of its maximum torque output, for efficient charging of the batterybank. Similarly, during braking or hill descent, the kinetic energy ofthe vehicle can be turned into stored electrical energy by regenerativebraking.

The hybrid drive train shown in the '970 patent has many advantages withrespect to the prior art which are retained by the present invention.For example, the electric drive motor is selected to be of relativelyhigh power, specifically, equal to or greater than that of the internalcombustion engine, and to have high torque output characteristics at lowspeeds; this allows the conventional multi-speed vehicle transmission tobe eliminated. As compared to the prior art, the battery bank,motor/generator, and associated power circuitry are operated atrelatively high voltage and relatively low current, reducing losses dueto resistive heating and simplifying component selection and connection.

It can thus be seen that while the prior art, including the '970 patent,clearly discloses the desirability of operating an internal combustionengine in its most efficient operating range, and that a battery may beprovided to store energy to be supplied to an electric motor in order toeven out the load on the internal combustion engine, there remainssubstantial room for improvement.

In particular, it is desired to obtain the operational flexibility of aparallel hybrid system, while optimizing the system's operationalparameters and providing a substantially simplified parallel hybridsystem as compared to those shown in the prior art, again as includingthe '970 patent.

As noted above, the present application is a continuation-in-part ofSer. No. 09/264,817, filed Mar. 9, 1999 (the 1817 application), whichdiscloses and claims several distinct improvements over the hybridvehicles shown in the '970 patent, as discussed in further detail below.similarly, the present application is a continuation-in-part of Ser. No.09/392,743, filed Sep. 9, 1999 (the 1743 application), which disclosesand claims several distinct improvements over the hybrid vehicles shownin the '970 patent and the '817 application, as discussed in furtherdetail below. The present application discloses and claims furtherimprovements over the vehicles of the '817 and '743 applications.

As discussed in detail below, the '817 and '743 applications (which arenot to be limited by this brief summary) disclose a new “topology” for ahybrid vehicle, wherein an internal combustion engine and a firstelectric “starting” motor, which can be operated as a starter, to startthe engine, a generator, to charge the battery bank responsive to torquefrom the engine or the wheels (i.e., during regenerative braking) or asa source of torque, to propel the vehicle, are connected to the roadwheels of the vehicle through a clutch, so that the engine can bedecoupled from the wheels during starting and battery charging, but canbe connected to the wheels to propel the vehicle. A second “traction”motor is directly connected to the road wheels to propel the vehicle.The vehicle operating mode is determined by a microprocessor responsiveto the “road load”, that is, the vehicle's instantaneous torque demands.The '743 application further discloses that a turbocharger may beprovided, and operated when needed to increase the torque output of theengine when torque in excess of its normally-aspirated capacity isrequired for more than a minimum time. The present application buildsfurther on these concepts.

Koide U.S. Pat. No. 5,934,395 and Schmidt-Brucken U.S. Pat. No.6,059,059 were addressed during the prosecution of the '817 application.Tsuzuki U.S. Pat. No. 6,018,198 and Werson U.S. Pat. No. 5,986,376 werealso each applied against one claim. As indicated, the '817 applicationdiscloses a hybrid vehicle comprising a controller, a battery bank, aninternal combustion engine, and two electric motors, a starting motorand a traction motor. The starting motor and engine are connected to theroad wheels through a clutch, while the traction motor is connecteddirectly and permanently to the road wheels for torque transmissiontherebetween, i.e., without a clutch therebetween. Koide does not showthis “topology” for a hybrid vehicle; although Koide does show a hybridvehicle having first and second motors along with an engine, thecomponents are not connected as described. Specifically, in Koide, bothmotors and the engine are connected to the road wheels by way of avariable-ratio transmission and a clutch, while, as noted, in the '817application only the combination of the engine and starting motor isconnected to the wheels through a clutch, while the traction motor isconnected directly to the wheels for torque transmission therebetween,that is, without a clutch or variable-ratio transmission. Morespecifically, Koide's entire disclosure is premised on being able tovary the ratios between the torque-producing components of his systemand the road wheels, in order that the engine can be smoothly startedwhen needed. According to the '817 application, only the starter motorand engine need to be disconnectible from the wheels for smoothstarting, while the traction motor can be connected to the road wheelsat all times. This represents a substantial simplification with respectto the system shown by Koide.

The Schmidt-Brücken patent also fails to show the topology shown in the'817 application. Schmidt-Brücken shows an engine 1 in combination witha starting motor 7, connected to the road wheels through a first clutch11, and a traction motor 19 connected to the road wheels through asecond clutch 23.

The '817 and '743 applications also disclose that the vehicle operatingmode is determined by a microprocessor responsive to the “road load”,that is, the vehicle's instantaneous torque demands, i.e., that amountof torque required to propel the vehicle at a desired speed. Theoperator's input, by way of the accelerator or brake pedals, or a“cruise control” device, indicates that continuing at steady speed isdesired, or that a change in vehicle speed is called for. For example,the operator's depressing the accelerator pedal signifies an increase indesired speed, i.e., an increase in road load, while reducing thepressure on the accelerator or depressing the brake pedal signifies adesired reduction in vehicle speed, indicating that the torque beingsupplied is to be reduced or should be negative. More particularly, itis important to note that the road load can vary between wide limits,independent of vehicle speed, and can be positive or negative, i.e.,when decelerating or descending a hill, in which case the negative roadload (that is, torque available at the wheels in excess of that requiredto propel the vehicle) is usually employed to charge the battery bank.

More particularly, it is important to recognize that road load is notthe same thing as vehicle velocity. Indeed, as noted, road load can benegative while vehicle velocity is positive, as during deceleration ordescent. Moreover, widely differing road loads may be encountered duringoperation at the same velocity; for example, operation at 50 mph on aflat road may involve a road load of only 30-40% of the engine's maximumoutput torque (MTO), while accelerating from the same speed whileclimbing a hill ma yinvolve a road load of well over 100% of MTO.

By the same token, control of the vehicle's operating mode in responseto monitoring of road load is not the same as controlling its operatingmode in response to vehicle speed. Numerous prior art references,including the Koide and Schmidt-Brucken patents, teach. the latter,i.e., indicate the vehicle operating mode should be controlled inresponse to vehicle speed. See Koide at col. 12, lines 45-48, andSchmidt-Brucken at col. 5, line 56-col. 6 line 29. Neither Koide norSchmidt-Brucken, nor any other reference of which the inventors areaware, recognizes that the desired vehicle operational mode shouldpreferably be controlled in response to the vehicle's actual torquerequirements, i.e., the road load. Doing so according to the inventionprovides superior performance, in terms of both vehicle response tooperator commands and fuel efficiency, under the widely-varyingconditions encountered in “real world” driving situations, than ispossible according to the prior art.

Moreover, as set forth in the '817 and '743 applications, in order toprovide maximum efficiency in use of fuel, it is essential to operatethe internal combustion engine of a hybrid vehicle only undercircumstances where the engine will be loaded so as to require at least30% of its maximum torque output (“MTO”) (it being understood throughoutthis specification and the appended claims that this 30% figure isarbitrary and can be varied). If the vehicle is controlled to shift intoan engine-only mode whenever it exceeds some arbitrary road speed, as inKoide and Schmidt-Brucken, it is apparent that the engine will beoperated at various times when the road load is less than 30% of MTO,for example, during deceleration or during descents. Moreover, as notedabove, the torque actually required can vary widely irrespective ofvehicle speed. For example, 30% of MTO may be sufficient to maintainsteady speed on a flat road, but 150% of MTO may be required foracceleration from the same speed. If the vehicle's operational mode isselected based solely on speed, as taught by Koide and Schmidt-Brücken,it will be incapable of responding to the operator's commands, and willultimately be unsatisfactory.

By comparison, according to the invention of the '817 and '743applications, and as further disclosed and claimed herein, the vehicle'soperating mode—that is, the selection of the source of torque needed topropel the vehicle—is determined based on the amount of torque actuallyrequired. In this way the proper combination of engine, traction motor,and starting motor is always available. This apparently-simple point hasevidently been missed entirely by the art.

Moreover, according to this aspect of the invention, the engine is usedto propel the vehicle only when it is efficient to do so. This is inaccordance with another aspect of the invention, wherein the engine isoperated only at high efficiency, leading directly to improved fueleconomy. For example, the engine is also used as needed to charge thebattery bank, e.g., in low-speed city driving, where the battery bankmay become depleted. The starter motor, which is operated as a generatorin these circumstances, is accordingly sized so as be able to accept atleast 30% of MTO as input torque; the battery bank is likewise sized soas to be able to accept a corresponding amount of charging current.Therefore the engine is never operated at less than 30% of MTO, and isthus never operated inefficiently. Koide and Schmidt-Brucken, becausethey teach switching the vehicle's operational mode based on vehiclespeed and not its torque requirements, would inherently operate theengine under less efficient conditions.

Furutani U.S. Pat. No. 5,495,906 discloses selection of operating modebased on a combination of vehicle speed and “vehicle load”; see, e.g.,col. 2, lines 39-47: “It is preferable that the running state detectionmeans detects vehicle speed and vehicle load . . . [and] that thecontrol means transfers the driving force generated by the engine to thepower generator and changes the electric power generated by the powergenerator [i.e., more of the engine power is used to charge thebatteries] in accordance with the vehicle load if the vehicle speed isthe predetermined value or less. Moreover, it is preferable to changethe predetermined value of the vehicle speed in accordance with thevehicle load.” It thus appears that Furutani determines the vehicleoperating state based on vehicle speed, although the change-over speedcan be varied responsive to the vehicle load. Furutani's “vehicle load”thus apparently includes the torque required to charge the battery, asdistinguished from applicants' “road load”, i.e., the torque required topropel the vehicle. Even assuming that Furutani's “vehicle load”, whichis not defined, were suggestive of “road load” as used by applicants,Furutani clearly does not suggest determining the operating mode basedon road load. More specifically, although Furutani recognizes adistinction between differing vehicle loads, and that the vehicle loadcan vary independent of vehicle speed, the vehicle operating mode isnonetheless selected based on vehicle speed; see col. 3, line 62-col. 4,line 32. Instead of varying the operating mode of the vehicle based onroad load, Furutani directs more or less of the engine's torque tobattery charging; see col. 4, lines 24-32.

Frank U.S. Pat. No. 6,054,844 shows several embodiments of hybridvehicles. In those where an engine is used to provide torque to thevehicle wheels, a continuously-variable transmission is employed, andthe ratio R is considered in determining the response to be made tooperator input, e.g., accelerator and brake pedal positions. Frank'scontrol strategy is to operate the engine along a line of optimalefficiency and use an electric motor to add to or subtract from theengine's output torque as appropriate. See col. 6, line 49-col. 7, line7 and col. 10, line 33—col. 11, line 22. Frank thus does not suggestcontrol of the vehicle operating mode responsive to road load.

U.S. Pat. No. 6,018,694 to Egami et al shows a controller for a hybridvehicle comprising an internal combustion engine and first and second“rotary electric units”. Although the question is not free from doubt,it appears from a detailed review of Egami's disclosure that torque fromthe engine is not supplied directly to the road wheels, but instead isused to drive one of the rotary electric units as a generator, in turnsupplying the second with current to provide torque for propelling thevehicle. Hence Egami does not show selection of the operational mode ofthe vehicle (that is, the determination whether propulsive torque is tobe provided from the engine, one or both of the motors, or all three) inresponse to the road load, since it does not appear that propulsivetorque is ever supplied from the engine to the wheels. Moreover, despitemaking reference to a “vehicle driving torque demand Mv*”, which mightbe misunderstood to be equivalent to applicant's road load, Egami infact does not determine the road load. More specifically, Mv* isdetermined by consulting a “map”, using “the vehicle speed V, theaccelerator lift ACC, the brake state BRK, and the shift position SFT asthe input parameters”. See col. 22, lines 23-26. The same point, i.e.,that the “vehicle driving torque demand Mv*” is not equivalent toapplicant's claimed road load, is made throughout Egami's extensivespecification; see, for example, col. 10, lines 28-32 and col. 27, lines58-65.

Deguchi U.S. Pat. No. 5,993,351 refers to decision-making regarding thevehicle mode of operation “based on the vehicle speed detected value andthe required motive force detected value” (Abstract; see also col. 1,line 41); the latter might be misunderstood to be equivalent to the roadload. Deguchi also states (col. 2, lines 7-9) that the vehicle “runs onthe motor at times of low load and runs on the internal combustionengine at times of high load”. However, Deguchi makes it clear that infact the operational-mode decision is made “based on the acceleratoraperture detected value θ which represents the required driving force ofthe vehicle and the detected vehicle speed” (col. 5, lines 19-21). Theaccelerator position and vehicle speed signals are the only relevantinputs to the vehicle controller shown in FIG. 2. Hence Deguchi does notshow controlling the vehicle operating mode responsive to road load asdefined by applicants.

Along generally similar lines, Boll U.S. Pat. No. 5,327,992 teaches ahybrid vehicle comprising a diesel engine and a motor on a common shaft,and intended to be operated such that the engine is only operatedefficiently, i.e., under relatively high load. The torque is required toovercome the “instantaneous tractive resistance” is determinedresponsive to the deflection of the accelerator pedal, i.e., in responseto operator command (see col. 3, line 13 and line 35); when this is lessthan the minimum amount of torque that can be produced efficiently bythe engine, the excess torque is used to power the motor as a generator.Boll also suggests that both the motor and engine can be used to propelthe vehicle when needed, e.g., during acceleration, and that the vehiclecan be operated in four different modes: (a) engine alone powering thevehicle; (b) motor only powering the vehicle, with the engine “generallyswitched off”; (c) engine and motor both powering the vehicle; and (d)engine powering vehicle, with excess torque powering motor in generatormode. Boll also teaches that a second motor can be provided, operable asa generator and then driven either by the engine directly or by exhaustgas, and that the resulting current can be used to charge the battery orto power the other motor.

Other references of interest are directed to the braking systems ofhybrid vehicles, see for example German patent 19 05 641 to Strifler,discussing a method of control of a braking system providing bothregenerative and mechanical braking, and the powering of ancillarysystems, such as power steering pumps, see U.S. Pat. No. 5,249,637 toHeidl. These references are discussed in further detail below withreference to improvements provided in these areas by the presentapplication.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an improved hybrid electricvehicle realizing substantially increased fuel economy and reducedpollutant emissions as compared to present day internal combustion andhybrid vehicles while suffering no significant penalty in performance,operating convenience, cost, complexity, or weight, which can beoperated efficiently by an operator accustomed to conventional vehicleswithout special training, and which does not require modification of theexisting infrastructure developed over the years to support conventionalvehicles.

More specifically, it is an object of the invention to provide such animproved vehicle that operates on fuel now widely available and usesbatteries already well understood and widely available, so that theoperator need not learn new driving techniques, deal with new fuelsupply arrangements, nor be obliged to be attentive to maintenance ofbatteries employing complex new technologies.

It is a more particular object of the present invention to provide animproved series-parallel hybrid electric vehicle wherein an internalcombustion engine and two separately-controlled electric motors canseparately or simultaneously apply torque to the driving wheels of thevehicle, controlled to realize maximum fuel efficiency at no penalty inconvenience, performance, or cost.

It is a further object of the invention to provide a series-parallelhybrid electric vehicle comprising two electric motors togetherproviding output power equal to at least 100 percent of the rated outputpower of the internal combustion engine, and more preferably up to about150-200 percent thereof, so that the engine operates under substantiallyoptimum conditions in order to realize substantial fuel economy andreduced emission of undesirable pollutants in operation.

More particularly, it is an object of the invention to provide aseries-parallel hybrid electric vehicle wherein the internal combustionengine is sized to efficiently provide the average power required foroperation at moderate and highway speeds, with two) or more)separately-controlled electric motors together sized to deliver theadditional power needed for acceleration and hill climbing.

Still another object of the invention is to provide a series-parallelhybrid electric vehicle wherein the electric motor and battery chargingcircuits operate at no more than about 30-50 amperes continuous current(although significantly greater currents may flow for short periods,under peak load conditions), whereby resistance heating losses aregreatly reduced, and whereby inexpensive and simple electricalmanufacturing and connection techniques can be employed.

It is a more specific object of the present invention to provide ahybrid drive system for vehicles that does not require the controllabletorque-transfer unit shown in the '970 patent, while providing thefunctional advantages of the hybrid vehicle shown in the '970 patent.

It is a more specific object of the invention to employ the controlflexibility provided by the improved hybrid drive train of the inventionto allow starting of the engine at comparatively high RPM, whilecontrolling the fuel/air mixture supplied during starting, throttlingthe engine, and providing a preheated catalytic converter, minimizingemission of unburned fuel and further improving fuel economy.

It is a more specific object of the invention to employ the controlflexibility provided by the improved hybrid drive train of the inventionto allow employment of a motor producing substantially constant torqueup to a base speed, and substantially constant power thereafter, as theengine starting motor, so that torque produced thereby can also be usedto propel the vehicle.

In addition to the above objects of the invention, which are similar tothose listed in the '817 and '743 applications, the invention of thepresent continuation-in-part application has as objects the broadeningof the useful ranges of loading of vehicles according to the invention,e.g., to provide highly efficient hybrid operation for a vehicle thatmay weigh 7,000 pounds empty but which can be loaded to weigh 10,000pounds or more, and may be expected to pull a trailer also weighing10,000 pounds or more.

A further object of the present invention is to provide furtherimprovements in methods of control of internal combustion engines forhybrid vehicles, to obtain very efficient use of fuel.

Another object of the present invention is to provide an optimal HVACsystem for hybrid vehicles.

Still a further object of the invention is to provide a braking systemfor hybrid vehicles including regenerative braking that provides optimaloperator feedback despite changes in operation responsive to the stateof charge of the battery bank.

Other aspects and objects of the invention will become clear as thediscussion below proceeds.

SUMMARY OF THE INVENTION

As discussed above, the '970 patent discloses hybrid vehicles wherein acontrollable torque transfer unit is provided capable of transferringtorque between an internal combustion engine, an electric motor, and thedrive wheels of the vehicle. See FIGS. 3-11 thereof. The direction oftorque transfer is controlled by a microprocessor responsive to the modeof operation of the vehicle, to provide highly efficient operation overa wide variety of operating conditions, and while providing goodperformance. The flow of energy—either electrical energy stored in asubstantial battery bank, or chemical energy stored as combustiblefuel—is similarly controlled by the microprocessor.

According to one aspect of the invention of the '817 and '743applications, which is also employed according to the presentcontinuation-in-part application, the controllable torque-transfer unitshown in the '970 patent is eliminated by replacing the single electricmotor shown therein by two separate motors, both operable as generatorsand as traction motors when appropriate. See FIGS. 3 and 4 hereof. As inthe '970 patent, an internal combustion engine is provided, sized toprovide sufficient torque to be adequate for the range of cruisingspeeds desired, and is used for battery charging as needed. The internalcombustion engine is connected to the drive wheels by a clutch operatedby the to microprocessor responsive to its selection of the vehicle'smode of operation in response to evaluation of the road load, that is,the vehicle's instantaneous torque demands and input commands providedby the operator of the vehicle. A relatively high-powered “traction”motor is connected directly to the output shaft of the vehicle; thetraction motor provides torque to propel the vehicle in low-speedsituations, and provides additional torque when required, e.g., foracceleration, passing, or hill-climbing during high-speed driving.

According to the invention of the '817 and '743 applications, arelatively low-powered starting motor is also provided, and can be usedto provide torque propelling the vehicle when needed. This second motoris connected directly to the internal combustion engine for starting theengine. Unlike a conventional starter motor, which rotates an internalcombustion engine at low speed (e.g., 60-200 rpm) for starting,necessitating provision of a rich fuel/air mixture for starting, thestarter motor according to the invention spins the engine at relativelyhigh speeds, e.g., 300-600 rpm, for starting; this allows starting theengine with a much less fuel-rich fuel/air mixture than is conventional,significantly reducing undesirable emissions and improving fuel economyat start-up. A catalytic converter provided to catalytically combustunburnt fuel in the engine exhaust is preheated to an effective workingtemperature before starting the engine, further reducing emissions.

In the embodiment discussed in detail, the starting motor is connecteddirectly to the engine, and this combination is connected to thetraction motor by a clutch for transfer of torque; the output shaft ofthe traction motor is then connected to the road wheels of the vehicle.In other embodiments, the engine/starting motor combination may beconnected to a first set of road wheels through a clutch, with thetraction motor connected to another set of road wheels directly; in afurther embodiment, plural traction motors may be provided. In eachcase, the engine is controllably disconnected from the road wheels bycontrol of the clutch. Engagement of the clutch is controlled by themicroprocessor, e.g., controlling an electrical or hydraulic actuator aspart of controlling the state of operation of the vehicle in response tothe road load.

For example, during low-speed operation, the clutch will be disengaged,so that the engine is disconnected from the wheels; the vehicle is thenoperated as a “straight” electric car, i.e., power is drawn from thebattery bank and supplied to the traction motor. Should the batteriesbecome relatively depleted (e.g., become discharged to 50% of fullcharge), the starter motor is used to start the internal combustionengine, which then runs at relatively high torque output (e.g., at leastabout 30% of its maximum torque), for efficient use of fuel, and thestarting motor is operated as a high-output generator to recharge thebattery bank.

Similarly, when the operator calls for more power than available fromthe traction motor alone, e.g., in accelerating onto a highway, thestarter motor starts the internal combustion engine; when it reaches anengine speed at which it produces useful torque, the clutch is engaged,so that the engine and starter motor can provide additional torque. Asnoted above, the engine is rotated at relatively high speed forstarting, so that the engine rapidly reaches a useful speed.

As in the '970 patent, the engine is sized so that it providessufficient power to maintain the vehicle in a range of suitable highwaycruising speeds, while being operated in a torque range providing goodfuel efficiency; if additional power is then needed, e.g., forhill-climbing or passing, the traction and/or starter motors can beengaged as needed. Both motors can be operated as generators, e.g., totransform the vehicle's kinetic energy into electrical power duringdescent or deceleration. Also as in the '970 patent, the peak power ofthe two motors together at least equals the rated power of the engine,as is necessary to provide good performance without employment of avariable-speed transmission or the equivalent.

In each of these aspects of the operation of the vehicle, and as in the'970 patent, the operator of the vehicle need not consider the hybridnature of the vehicle during its operation, but simply provides controlinputs by operation of the accelerator and brake pedals. Themicroprocessor determines the appropriate state of operation of thevehicle based on these and other inputs and controls the variouscomponents of the hybrid drive train accordingly.

It is also within the scope of the invention to operate one or both ofthe motors at differing rotational speeds than the engine, so that eachcan be optimized for the demands thereon. More specifically, motors canin general be made smaller if they can be operated at relatively highRPM. Motors operating at up to 9000-18,000 RPM appear appropriate forthe present application. However, operating the internal combustionengine at this speed would likely lead to undesirable levels of noiseand vibration, and might constrain its performance characteristics in anundesirable manner. Accordingly, for example, the starter motor mightdrive the engine through a pinion geared to a larger toothed flywheel,as conventional. Similarly, it might be desirable to provide thetraction motor as a relatively high-speed unit, driving the road wheelsthrough a chain, belt, or gear reduction unit. The starter motor may beconfigured as a “faceplate” or “pancake” motor, essentially forming theflywheel of the engine, and rotating at engine speed, while the tractionmotor is a much higher speed induction motor connected to the vehicledriveshaft by a chain drive reduction unit. It is also within the scopeof the invention, as noted above, to operate the engine and the twomotors at the same speed when the clutch is engaged, avoidingintermediate gear trains or like mechanical components and the attendantcost, complexity, weight, audible noise, and frictional lossesoccasioned by their use.

Other improvements provided according to the invention include providingthe batteries in two series-connected battery banks, with the vehiclechassis connected to the batteries at a central point, between thebanks. This “center-point-chassis” connection reduces the voltagebetween various circuit components and the vehicle chassis by half,significantly reducing the electrical insulation required andsimplifying such issues as heat-sinking of power semiconductors used inthe inverter circuitry. Providing dual battery banks and dual electricmotors, as above, also provides a degree of redundancy, permittingcertain component failures without loss of vehicle function.

In the preferred embodiment, both the traction and starting motors areAC induction motors of four or more phases and the accompanying powercircuitry provides current of more than three, preferably five, phases,allowing the vehicle to function even after failure of one or morecomponents. These motors, and the inverter/chargers driving them, shouldbe chosen and operated such that the motors have torque outputcharacteristics varying as a function of rpm as illustrated in FIG. 14of the '970 patent; that is, the motors should produce substantiallyconstant torque up to a base speed and should produce substantiallyconstant power at higher speeds. The ratio of the base. to maximum speedcan vary between about 3 to 1 and about 6 to 1. By comparison, theseries-wound DC motors conventionally used as engine starting motorsprovide very high torque, but only at very low speeds; their torqueoutput drops precipitously at higher speeds. Such conventional startermotors would be unsatisfactory in the present system.

During substantially steady-state operation, e.g., during highwaycruising, the control system operates the engine at varying torqueoutput levels, responsive to the operator's commands. The range ofpermissible engine torque output levels is constrained to the range inwhich the engine provides good fuel efficiency. Where the vehicle'storque requirements exceed the engine's maximum efficient torque output,e.g., during passing or hill-climbing, one or both of the electricmotors are energized to provide additional torque; where the vehicle'storque requirements are less than the minimum torque efficientlyprovided by the engine, e.g., during coasting, on downhills or duringbraking, the excess engine torque is used to charge the batteries.Regenerative charging may be performed simultaneously, as torque fromthe engine and the vehicle's kinetic energy both drive either or bothmotors in generator mode. The rate of change of torque output by theengine may be controlled in accordance with the batteries' state ofcharge.

The vehicle is operated in different modes, depending on itsinstantaneous torque requirements, and the state of charge of thebattery, and other operating parameters. The mode of operation isselected by the microprocessor in response to a control strategydiscussed in detail below; the values of the sensed parameters inresponse to which the operating mode is selected may vary depending onrecent history, or upon analysis by the microprocessor of trips repeateddaily, and may also exhibit hysteresis, so that the operating mode isnot repetitively switched simply because one of the sensed parametersfluctuates around a defined setpoint.

None of the implementations of the invention shown in the '970 patent orthe '817 and '743 applications include a conventional multi-speedtransmission between the motors and engine and the road wheels, and itwas stated that a desirable aspect of the invention was to avoid suchtransmissions, so that the rotational speeds of the two motors and theengine were fixed with respect to one another, and to the speed of theroad wheels. However, it now appears that in some circumstances atwo-speed transmission may be desired in some cases to broaden the rangeof utility of the vehicles of the invention (principally to extend theirload-carrying capabilities) while still providing highly efficientoperation, and to include such a two-speed transmission is accordinglypart of the invention of the present continuation-in-part application.Such a two-speed transmission could be operated infrequently as atwo-speed “range selector”, or could be operated essentially as aconventional automatic transmission, that is, be repetitively shiftedduring acceleration, upon “kick-down” and the like.

More specifically, it is of great present interest to optimize thehybrid power train of the invention for use with relatively heavyvehicles, such as vans, pickup trucks and “sport-utility vehicles”(SUVs). Such vehicles have become increasingly popular in recent years,despite their generally poor fuel mileage; it would be highly desirableto provide vehicles with generally similar load-carrying abilities andperformance with better fuel economy. Still more particularly,heretofore large classes of such vehicles have not been subject tocertain emission regulations; however, such regulations are expected totake effect shortly. Accordingly, it would be very desirable to providesuch vehicles with hybrid power trains that will allow their owners toenjoy the load-carrying and performance abilities of the existingvehicles with improved fuel economy and reduced emissions.

One of the aspect of SUVs and similar vehicles that must be consideredin design of a suitable hybrid powertrain is that their owners use themto carry and tow widely-varying loads. That is, a conventional SUV mightweigh 5,500 pounds, and might typically be used during the week totransport a 140 pound person, up to 300 pounds of children, and 50pounds of groceries. However, on the weekend the family might load thevehicle with half a ton of camping gear and the like and set off for themountains towing a 7,500 pound trailer. The vehicle must. provideadequate acceleration, passing, and hill-climbing performance in bothuses. In order to have sufficient power at times of maximum loading, thevehicle is grossly overpowered under all different circumstances; thatis, only when the vehicle is laden to near-maximum capacity and pullingup a long hill does the engine deliver near maximum torque for anylength of time. Under all other circumstances, it is run veryinefficiently, as noted in connection with FIGS. 1 and 2 (reproducedherein from the '970 patent).

An important aspect of the invention as described by the presentcontinuation-in-part application as well as the predecessor applicationsand the '970 patent lies in controlling the operation of the internalcombustion engine of a hybrid vehicle so that it is only operated athigh efficiency, that is, only when is it loaded to require asubstantial fraction e.g., 30% of its maximum torque output. That is,the engine is never run at less than 30% of maximum torque output(“MTO”). As discussed in the '970 patent and the '817 application, thiscan be accomplished by sizing the engine so that it can efficientlypropel the vehicle unassisted at highway speeds; if additional torque isrequired for passing or hill-climbing, the traction motor is operated.Application Ser. No. 392,743 further adds the idea of providing aturbocharger, controlled by the microprocessor only to operate whentorque in excess of the engine's rated normally-aspirated maximum torqueoutput (MTO) is needed for an extended period of time, for example intowing a trailer. By employing the turbocharger only when actuallyneeded, many of the drawbacks inherent in conventional turbocharger usesare eliminated. Typically the turbocharger may be sized such that theengine provides up to 150% of MTO when turbocharged.

According to one aspect of the invention of the presentcontinuation-in-part application, the range of efficient use of thehybrid vehicle of the invention is further broadened by providing atwo-speed transmission between the engine and road wheels, so as toallow variation in the overall gear ratio and therefore vary the amountof torque available at the wheels. As noted above, this could be amanually- or automatically-operated “range shifting” gearbox akin tothose presently provided on SUVs and the like, to allow shifting into a“low range”, for example, when a heavy trailer is be towed, or could beoperated similarly to a conventional multispeed transmission, that is,to provide a sequence of effective overall gear ratios each time thevehicle is accelerated.

A further improvement made according to the present continuation-in-partapplication has to do with the braking system. As noted above, the '970patent (as well as numerous other prior art references) discloseregenerative braking, that is, employing the microprocessor to controlthe operation of inverter/chargers connected between the motor andbattery bank so that when the operator desires to slow the vehicle, itsmomentum is used to drive the motor in generator mode, charging thebattery. There are certain limitations on this as a method of vehiclebraking, which must be addressed by any useful vehicle. In particular, ahydraulic braking system of generally conventional design must beprovided for several reasons: first, for safety, in the event that theregenerative system fails for any reason; second, to provide braking inthe event the battery bank is fully charged and cannot accept furthercharge (since overcharging is highly detrimental to battery life); andto provide braking when regenerative braking is not available, e.g.,when at a standstill. The present application discloses certainimprovements in hydraulic braking systems desired to optimize theirdesign for use with hybrid vehicles, as well as a mechanism providingoptimized brake “feel” to the driver, regardless whether conventional,regenerative, or both braking systems are in use.

The present application also discloses certain problems inherent inapplication of conventional vehicles' heating, ventilation and airconditioning systems to hybrid vehicles, and describes preferredsolutions to these problems.

A further improvement according to the present invention includes theprovision of an auxiliary 12 volt supply system, allowing the hybridvehicle of the invention to “jumpstart” another vehicle, or likewise tobe jumpstarted as might be necessary after a long hiatus, and to allowuse of conventional 12 volt accessories, such as radios and otherelectronic items.

The present application also discloses further useful modifications andenhancements to the hybrid vehicles of the predecessor applications.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood if reference is made to theaccompanying drawings, in which:

FIG. 1 is a plot of output power versus rotational speed (RPM) for atypical internal combustion engine, illustrating the relative fuelconsumption of the engine as used in a conventional automobile ingallons/horsepower-hour;

FIG. 2 is a similar plot describing operation of a relatively smallinternal combustion engine used in the present invention undercircumstances similar to those depicted in FIG. 1;

FIG. 3 shows a schematic diagram of the principal components of a firstembodiment of the hybrid vehicle drive system according to theinvention;

FIG. 4 shows a block diagram of the principal components of the drivesystem of the invention in a second embodiment, differing in certainmechanical arrangements from that of FIG. 3, and illustrating variouscontrol signals provided in both embodiments;

FIG. 5 shows a partial schematic diagram of the battery bank, inverter,and motor circuitry;

FIG. 6 is a diagram illustrating differing modes of vehicle powertrainoperation, plotted on a three dimensional chart, illustrating that themode of vehicle operation is a function of the state of charge of thebattery bank, the instantaneous road load, and time;

FIG. 7, comprising FIGS. 7 (a)-(c), and extending over two sheets, is atiming diagram showing road load, engine torque output, the state ofcharge of the battery bank, and engine operation as functions of time,thus illustrating a typical control strategy employed during low-speedcity driving, highway cruising, and extended high-load driving;

FIG. 8, comprising FIGS. 8 (a)-(d), are diagrams indicating the flow oftorque and of energy among the components of the hybrid powertrain ofthe invention, in various modes of operation;

FIG. 9 is a simplified flow chart of the algorithm employed by themicroprocessor to implement the control strategies provided by thevehicle according to the invention;

FIG. 9( a) is a flow chart of an engine starting subroutine employed inthe flowchart of FIG. 9;

FIG. 9( b) is an alternate version of one of the steps of the flowchartof FIG. 9, implementing a modification to the vehicle control strategy;

FIG. 9( c) is an alternate version of another of the steps of theflowchart of FIG. 9, similarly implementing a modification to thevehicle control strategy;

FIG. 10 illustrates the preferred torque versus speed characteristics ofthe electric starting and traction motors, and of the internalcombustion engine;

FIG. 11 is a schematic diagram similar to FIG. 3, illustrating analternative embodiment of the hybrid vehicle powertrain according to theinvention, wherein the engine is provided with a turbocharger which iscontrollably operable, so as to be employed only when needed;

FIG. 12 is a three-dimensional diagram comparable to FIG. 6, showing themodes of operation of the turbocharged hybrid vehicle of FIG. 11;

FIG. 13 is a timing diagram similar to FIG. 7, again comprising FIGS.13( a)-(c), extending over two sheets, and illustrating typicaloperation of the turbocharged hybrid vehicle of FIG. 11;

FIG. 14 is a schematic diagram similar to FIGS. 3 and 11, illustrating afurther alternative embodiment of the hybrid vehicle powertrainaccording to the invention, wherein a second traction motor is connectedto a second set of road wheels, providing a particularly convenient wayof providing four-wheel drive;

FIG. 15 is a schematic diagram of the preferred brake system of a hybridvehicle according to the invention; and

FIG. 16 is a is a schematic diagram of the preferred heating,ventilation and air conditioning system of a hybrid vehicle according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring specifically to FIG. 1, which is reproduced here from the '970patent for convenience, curve 10 represents the output power versusengine speed (RPM) of a typical spark ignition gasoline-fueled internalcombustion engine as used with an automatic transmission in a typicalsedan of 3,300 pounds. As can be seen, the maximum engine poweravailable is about 165 horsepower at about 5,000 RPM. Also shown in FIG.1 by the curve labeled “Large Car Average Power Requirements” are theaverage power requirements of such a vehicle. Points C, S, and H on thiscurve show average fuel consumption in city, suburban, and highwaydriving, respectively; in particular, point C shows that the averagepower required in typical city driving is less than 5 hp. Point S showsthat the average power consumed in suburban driving is 10 hp, and pointH shows that the power needed for steady-speed highway driving is onlyabout 30 hp. Thus, the vehicle is vastly overpowered at all times exceptduring acceleration or hill-climbing.

FIG. 1 also includes dashed-line curves indicating the relative fuelconsumption of the engine. As can be seen, reasonable fuel efficiency,that is, at least about 105 percent relative fuel consumption (100%being ideal), is reached only when the engine is operated at betweenabout 2,000 and 4,000 RPM, and when producing between about 75 and 150horsepower. FIG. 1 thus indicates that the typical internal combustionengine is operated with reasonable efficiency only when producingbetween about 50 and about 90% of its maximum output power. The typicalautomobile only requires such substantial power under conditions ofextreme acceleration or hill climbing.

Accordingly, it will be appreciated that the typical engine is operatedefficiently only during relatively brief intervals; more specifically,at lower power outputs, losses due to friction and pumping consumelarger fractions of the engine's total torque, so that a lower fractionis available to propel the vehicle. As can be seen, during typicalhighway driving, shown by point H, the relative fuel consumption is onthe order of 190 percent of that required during the most efficientoperation of the engine. The situation is even worse in suburbandriving, where the relative fuel consumption is nearly 300 percent ofthe most efficient value, and in city driving, where the relative fuelconsumption is almost 350 percent of that required at most efficientoperation.

FIG. 1 thus demonstrates that an internal combustion engine havingsufficient horsepower for adequate acceleration and hill climbingcapability must be so oversized with respect to the loads encounteredduring most normal driving that the engine is grossly inefficient in itsconsumption of fuel. As noted, FIG. 1 further shows that only about 30horsepower is needed to cruise on the highway even in a relatively largecar.

FIG. 2 (again reproduced from the '970 patent for convenience) issimilar to FIG. 1, and illustrates. the operational characteristics ofthe same 3,300 pound car if driven by a relatively small engine having amaximum horsepower rating of about 45 horsepower at 4,000 RPM. The powerrequirement of the vehicle during highway cruising, shown by point H onthe curve marked “Large Car Average Power Requirements”, is in thecenter of the most efficient region of operation of the engine. However,even with this small engine thus optimized for highway cruising, thereis a substantial gap. between the “Engine Operating Power” curve and theAverage Power Requirement curve 14. That is, even this small engineproduces substantially more power at low RPM than needed for citydriving (point C) or for suburban driving (point S). Accordingly, evenwith a small engine sized appropriately for highway cruising,substantial inefficiencies persist at lower speeds. Moreover, of course,such a vehicle would have unsatisfactory acceleration and hill climbingability. Therefore, the answer is not simply to replace large internalcombustion engines with smaller internal combustion engines.

The prior art recognizes that there are substantial advantages to begained by combining the virtues of a gasoline or other internalcombustion engine with those of an electric motor running from a batterycharged by the internal combustion engine. However, the prior art hasfailed to provide a solution which is directly price- andperformance-competitive with vehicles now on the market; moreover, inorder that such a vehicle can be commercially successful, it must alsobe no more complex to operate than existing vehicles.

As indicated above, “straight” electric vehicles, that is, vehicleshaving electric traction motors and batteries requiring recharge at theend of each day's use, do not have sufficient range and require too muchtime to recharge to fully replace conventional automobiles. Further, theoperational costs of such vehicles are not competitive with internalcombustion vehicles operated on fuels derived from renewable resourcessuch as ethanol, and are even less competitive with gasoline-fueledautomobiles.

A first type of series hybrid vehicles, involving a gasoline enginedriving a generator charging a battery powering an electric tractionmotor, are limited in acceleration and hill climbing ability unless theelectric motor is made very large, costly, and bulky. The alternativeseries hybrid approach, involving a transmission between a relativelysmaller electric motor and the wheels to provide the torque needed toaccelerate quickly, loses the virtue of simplicity obtained byelimination of a multi-speed transmission. These vehicles fail torealize the advantages provided by the parallel hybrid system in whichboth an internal combustion engine and an electric motor provide torqueto the wheels as appropriate.

However (apart from the '970 patent) the prior art relating to parallelhybrid vehicles fails to disclose a system sufficiently simple foreconomical manufacture. The art further has failed to teach the optimummethod of operation of a parallel hybrid vehicle.

Moreover, the art relating to parallel hybrids (again, apart from the'970 patent) does not teach the appropriate operational parameters to beemployed, relating to the relative power outputs of the internalcombustion engine and the electric motor; the type of electric motor tobe employed; the frequency, voltage, and current characteristics of themotor/battery system; the proper control strategy to be employed undervarious conditions of use; and combinations of these.

As shown in the '970 patent with reference to FIGS. 1 and 2 thereof, andagain above, typical modern automobiles operate at very low efficiency,due principally to the fact that internal combustion engines are veryinefficient except when operating at near peak torque output; thiscondition is only rarely met. (The same is true, to greater or lesserdegree, of other road vehicles powered by internal combustion engines.)According to an important aspect of the invention of the '970 patent,substantially improved efficiency is afforded by operating the internalcombustion engine only at relatively high torque output levels,typically at least 35% and preferably at least 50% of peak torque. Whenthe vehicle operating conditions require torque of this approximatemagnitude, the engine is used to propel the vehicle; when less torque isrequired, an electric motor powered by electrical energy stored in asubstantial battery bank drives the vehicle; when more power is requiredthan provided by either the engine or the motor, both are operatedsimultaneously. The same advantages are provided by the system of thepresent invention, with further improvements and enhancements describedin detail below.

According to one aspect of the invention of the '970 patent, theinternal combustion engine of a hybrid vehicle is sized to supplyadequate power for highway cruising, preferably with some additionalpower in reserve, so that the internal combustion engine operates onlyin its most efficient operating range. The electric motor, which issubstantially equally efficient at all operating speeds, is used tosupply additional power as needed for acceleration and hill climbing,and is used to supply all power at low speeds, where the internalcombustion engine is particularly inefficient, e.g., in traffic.

As indicated above, this application discloses certain modifications,improvements, and enhancements of the hybrid vehicles shown in U.S. Pat.No. 5,343,970; where not otherwise stated, the design of the vehicle ofthe present invention is similar to that shown in the '970 patent.Components commonly numbered in this application and the '970 patent arefunctionally similar, with detail differences as noted. The advantagesof the system shown in the '970 patent with respect to the prior art areprovided by that of the present invention, with further improvementsprovided by the latter, as detailed herein.

In the system of the '970 patent, torque from either or both the engineand motor is transferred to the drive wheels of the vehicle by acontrollable torque-transfer unit. This unit also allows torque to betransferred between the motor and engine, for starting the engine, andbetween the wheels and motor, for regenerative battery charging duringdeceleration of the vehicle. This unit, while entirely practical,comprises gears for power transfer, which are inevitably a source ofaudible noise and frictional losses. According to one aspect of thepresent invention, the controllable torque-transfer unit is eliminated.Instead, two electric motors are provided, each separately controlled bya microprocessor controller responsive to operator commands and sensedoperating conditions.

In this connection, it will be understood that the terms“microprocessor” and “microprocessor controller” are usedinterchangeably throughout the present application, and it is to befurther understood that these terms as used herein include various typesof computerized control devices not always referred to as“microprocessors” per se, such as computers themselves incorporatingmicroprocessors, digital signal processors, fuzzy logic controllers,analog computers, and combinations of these. In short, any controllercapable of examining input parameters and signals and controlling themode of operation of the vehicle according to a stored program, asdiscussed below in detail, is considered to be a “microprocessor” or“microprocessor controller” as used herein. Furthermore, the electronicfuel injection and electronic engine management devices shown in FIGS. 3and 4 as separate elements might also be integrated within the“microprocessor” or “microprocessor controller” as described herein.

FIG. 3 of the present application shows a first embodiment of thepresent invention, while FIG. 4, discussed below, shows a secondembodiment illustrating certain alternative mechanical arrangements;overall the two embodiments are very similar, and functionally they aresubstantially identical. FIG. 11, also discussed below, illustrates afurther embodiment, and FIG. 14 incorporates still further improvements.

In the FIG. 3 embodiment, a traction motor 25 is connected directly tothe vehicle differential 32, and thence to the road wheels 34. Astarting motor 21 is connected directly to the internal combustionengine 40. The motors 21 and 25 are functional as motors or generatorsby appropriate operation of corresponding inverter/charger units 23 and27, respectively, connected between the motors and battery bank 22. Atpresent, essentially conventional lead-acid batteries are preferred forbattery bank 22, since these are widely available and well understood.More advanced batteries may be used if and when they become widelyavailable and economically competitive.

Motors 21 and 25 are controllably connected for torque transfer by aclutch 51, mechanically interlocking the shafts 15 and 16 of motors 21and 25 respectively. As discussed further below in connection with FIG.4, microprocessor (“μP”) 48 is provided with signals indicative of therotational speeds of shafts 15 and 16, and controls operation of engine40, motor 21, and motor 25 as necessary to ensure that the shafts arerotating at substantially the same speed before engaging clutch 51.Accordingly, clutch 51 need not necessarily be an ordinary automotivefriction clutch (as illustrated schematically in FIG. 1), asconventionally provided to allow extensive relative slipping before theshafts are fully engaged. More particularly, as slipping of clutch 51 isnot required to propel the vehicle initially from rest, as is the casein conventional vehicles, clutch 51 need not allow for extensiveslipping when being engaged. In some cases it may be satisfactory toprovide clutch 51 as a simple self-aligning mechanical interlock (asshown in FIG. 4), wherein positive mechanical connection is made betweenthe shafts 15 and 16 upon engagement. Such a mechanical interlock ismuch simpler and less expensive than a friction clutch. In either case,clutch 51 is operated by microprocessor 48, e.g., through a knownelectric or hydraulic actuator 53, together with the other components ofthe system, in. accordance with the operational state of the vehicle andthe operator's input commands.

The respective positions of motor 21 and engine 40 with respect toclutch 51, motor 25, and wheels 34 could be reversed as compared totheir positions in FIGS. 3 and 4 without affecting the function of thesystem, although as engine 40 would then require torque transmittingconnection at both ends of its crankshaft, some additional complexitywould result.

As shown in FIG. 4, shaft encoders 18 and 19 may be mounted on theshafts 15 and 16 of starting motor 21 and traction motor 25,respectively, to provide signals to microprocessor 48 indicative of therelative rotational speeds of the shafts, and their respectiverotational positions. Such shaft encoders are well-known andcommercially available. Alternatively, signals indicative of therotational speeds of the shafts may be derived from the inverter controlsignals, in accordance with well-known principles of control of“sensorless” motor drives (see, for example, Bose, “Power Electronicsand Variable Frequency Drives”, IEEE, 1996). However, provision ofencoders 18 and 19 will allow better low-speed torque characteristics ofmotor 21 and 25, and thus reduction in cost.

Thus being provided with signals indicative of the rotational speeds ofshafts 15 and 16, microprocessor 48 controls operation of engine 40,motor 21, and motor 25 as necessary to ensure that the shafts arerotating at substantially the same speed before engaging clutch 51;therefore, clutch 51 need not be an ordinary automotive friction clutch(as illustrated schematically in FIG. 3), as conventionally provided toallow extensive slipping before the shafts are fully engaged. Accordingto this aspect of the invention, and particularly if microprocessor 48is made capable of ensuring that shafts 15 and 16 bear a desiredrelative angular relationship, clutch 51 instead may be a simple,relatively inexpensive self-aligning mechanical interlock (asillustrated schematically in FIG. 4), wherein positive mechanicalconnection is made between the shafts 15 and 16 upon engagement.

FIG. 4 also shows additional signals provided to microprocessor 48 inboth the FIG. 3 and the FIG. 4 embodiments. These include operator inputcommands, typically acceleration, direction, deceleration, and “cruisemode” commands, as shown. The acceleration and deceleration commands maybe provided by position-sensing encoders 71 and 72 (FIG. 3) (which couldbe configured as rheostats, Hall-effect sensors, or otherwise) connectedto microprocessor 48 by lines 67 and 68, to inform the microprocessor ofthe operator's commands responsive to motion of accelerator and brakepedals 69 and 70 respectively. The microprocessor monitors the rate atwhich the operator depresses pedals 69 and 70 as well as the degree towhich pedals 69 and 70 are depressed. The operator may also provide a“cruise mode” signal, as indicated, when a desired cruising speed hasbeen reached. The microprocessor uses this information, and othersignals provided as discussed herein, in accordance with the operationalstrategy discussed in detail below in connection with FIGS. 6-9, toproperly control operation of the vehicle according to the invention byappropriate control signals provided to its various components.

For example, suppose the vehicle has been operated in city traffic forsome time, that is, under battery power only. Typically the operatorwill only depress the accelerator pedal 69 slightly to drive in traffic.If the operator then depresses accelerator pedal 69 significantlyfarther than he or she had, for example, the prior few timesacceleration was required, this may be taken as an indication that anamount of torque that can efficiently be provided by engine 40 willshortly be required; microprocessor will then initiate the sequencewhereby starting motor 21 will be used to start engine 40.

Upon initiation of the engine starting sequence, a heater 63 (FIG. 3)will first be used to preheat a catalytic converter 64 provided in theengine exhaust system 62, so that any fuel that is not combusted duringstarting and subsequent running of the engine 40 will be catalyticallycombusted, reducing emission of undesirable pollutants. A temperaturesensor 102 is preferably provided, so as to ensure the engine is notstarted until the catalytic material is heated to effective workingtemperature. As noted above, engine starting is preferably performedwith the engine turning at a higher speed than is conventional, so thata the fuel/air ratio need only be slightly (e.g., 20%) richer thanstoichiometric. As a result, only very limited amounts of pollutants areemitted during engine starting. By comparison, in conventional vehicles,a very significant fraction of the total pollutants emitted during anygiven trip are emitted during the first 30-60 seconds of operation, dueto the extremely rich mixtures normally supplied during starting, and tothe ineffectiveness of the catalyst until it has been heated by theexhaust.

If the operator depresses the pedal 69 rapidly, indicating an immediateneed for full acceleration, the preheating step may be omitted; however,a preferable alternative may be to allow the traction and startingmotors to be driven at or slightly beyond their rated power, providingadequate torque, for a short time sufficient to allow the catalyst to bewarmed and the engine started.

Similarly, if the operator depresses the brake pedal 70 relativelygently, all braking may be provided by regenerative charging of thebatteries; if the operator instead presses aggressively on brake pedal70, and/or presses brake pedal 70 beyond a predetermined point, bothmechanical and regenerative braking will be provided. Mechanical brakingis also provided on long downhills when the batteries are fully charged,and in case of emergency. Further aspects of the preferred brake systemof the hybrid vehicles of the invention are added by the presentcontinuation-in-part application, and are discussed below.

In addition to engine and starting motor speed and traction motor speed,monitored by shaft encoders 18 and 19 as discussed above, batteryvoltage, battery charge level, and ambient temperature are also eithermonitored directly or derived from monitored variables. In response tothese inputs, and the operator inputs, microprocessor controller 48operates a control program (see the high-level flowchart of an exemplarycontrol program provided as FIG. 9), and provides output control signalsto engine 40, by commands provided to its electronic fuel injection unit(EFI) 56 and electronic engine management system (EEM) 55, and tostarting motor 21, clutch 51, traction motor 25, inverter/charger units23 and 27, and other components.

As indicated in FIG. 4, the control signals provided toinverter/chargers 23 and 27 by microprocessor 48 allow control of thecurrent (represented as I), of the direction of rotation of the motor 25(represented as +/−), allowing reversing of the vehicle, and of thefrequency of switching (represented as f), as well as control ofoperation of the motors 21 and 25 in motor or generator mode.Inverter/chargers 23 and 27 are separately controlled to allowindependent operation of motors 21 and 25. Inverter/charger operation isdiscussed further below in connection with FIG. 5.

As noted above, the FIGS. 3 and 4 embodiments of the system of theinvention differ in certain mechanical arrangements, intended toillustrate variations within the scope of the invention, and FIG. 4 alsoprovides more detail concerning the specific control signals passingbetween various elements of the system.

Referring to the differing mechanical arrangements, it will be observedthat in FIG. 3 the shafts of motors 21 and 25 are illustrated as coaxialwith that of engine 40; this is the simplest arrangement, of course, butwould require the engine 40 and starter motor 21 to rotate at the samespeed at all times, and at the same speed as traction motor 25 whenclutch 51 is engaged. As noted above, it may be preferable to designmotors 21 and 25 to have maximum speeds of 9000-15,000 rpm, so that theycould be made smaller, lighter, and less costly than slower-rotatingmotors. However, it is envisioned that a preferred maximum speed forengine 40 is 6000 rpm, as internal combustion. engines running atsubstantially higher speeds wear rapidly and tend to have limited torqueat low speed, and because higher frequency engine noise and vibrationcan also be difficult to absorb. It is within the scope of the inventionto provide the motors coaxial with the engine shaft, as illustrated inFIG. 3, but to provide a planetary gearset(s) between the shafts ofeither or both of traction motor 25 and starting motor 21 and the outputshaft to permit differing engine and motor speeds. Further alternativesto this aspect of the invention are again added by the presentcontinuation-in-part application, and are discussed below.

FIG. 4 illustrates an alternative construction, also permittingdiffering engine and motor speeds. In this case, the output shaft ofstarting motor 21 is shown connected to that of engine 40 by spur gears52, and traction motor 25 is connected to the output shaft 55 by chaindrive indicated at 54. Numerous other arrangements will occur to thoseof skill in the art. However, in each case there is no variable-ratiotransmission between the sources of torque—that is, the motors 21 and25, and the engine 40—and the road wheels 34. Again, furtheralternatives to this aspect of the invention are added by the presentcontinuation-in-part application, and are discussed below.

It is also within the scope of the invention to connect the tractionmotor to one set of wheels, and to connect the combination of the engine40 and starting motor 21 to another set of wheels through clutch 51,thus providing a four-wheel drive vehicle with differing power sourcesfor the alternate pairs of wheels. In this embodiment, the torque fromthe traction motor 25 is effectively combined with that from engine 40(and from starting motor 21, when used as a source of propulsive torque)by the road surface, rather than by mechanical connection, as in theFIGS. 3 and 4 embodiment. A further alternative would be to provide acomplete system as in FIG. 3 driving one pair of road wheels, and aseparate traction motor driving a second pair of road wheels. Bothembodiments are within the scope of the invention, and the controlstrategy is essentially the same as to both. See FIG. 14 and the relatedtext below for further discussion.

Other elements of the system as illustrated in FIGS. 3 and 4 aregenerally as discussed in the '970 patent, including supply of fuel 36from tank 38, air filter 60, and throttle 61.

Control of engine 40 by microprocessor 48 is accomplished by way ofcontrol signals provided to electronic fuel injection (EFI) unit 56 andelectronic engine management (EEM) unit 55, responsive to throttleoperation; preferably, the throttle in turn is operated electronicallyresponsive to the opertor's depression of the accelerator pedal. Controlof starting of engine 40, and using either or both of starting motor 21and traction motor 25 as motors, providing propulsive torque, or asgenerators, providing recharging current to battery bank 22, isaccomplished by microprocessor 48 by way of control signals provided toinverter/charger units 23 and 27.

Under deceleration, for example, during descents, or as needed forbraking, or when the engine's instantaneous torque output exceeds thevehicle's current torque requirements, either or both of motors 21 and25 are operated as generators, providing regenerative recharging ofbattery bank 22. FIG. 7, discussed below, illustrates this aspect of theoperation of the vehicle of the invention in further detail.

Thus, as indicated above, when microprocessor 48 detects a continuedoperator requirement for additional power, such as during transitionfrom slow-speed to highway operation, or by measuring the rate at whichthe operator depresses accelerator pedal 69, engine 40 is started usingstarter motor 21 and brought up to speed before clutch 51 is engaged, toensure a smooth transition. As cruising speed is reached (as determinedby monitoring the operator's commands), power to traction motor 25 (andto starter motor 21, if also used to accelerate the vehicle) isgradually reduced. Provision of the clutch 51 and separate starter motor21, as compared to using the single traction motor to start engine 40while simultaneously accelerating the vehicle, that is, as in the '970patent, simplifies the control arrangements somewhat.

In one possibly preferred embodiment, both motors 21 and 25 and clutch51 may be provided in a single sealed housing, possibly bathed in oilfor cooling and protection from dust and the like. It is also known tocontrol auxiliary motors, such as conventional starter motors, to absorbor add torque to that provided by an associated internal combustionengine, to damp out vibration caused by fluctuation of the torqueprovided by the engine; doing so herein using either or both of motors21 and 25 is within the scope of the invention, and is simplified byvirtue of the direct connection of the engine 40 to the drive wheelsthrough motors 21 and 25 according to the invention.

Provision of the clutch 51 and separate starter motor 21 also allowsanother important improvement to be provided according to the presentinvention, namely starting engine 40 at high speed, e.g., about 300-600rpm, as compared to the 60-200 rpm starts conventionally provided. As isgenerally known in the art (see Simanaitis, “What goes around comesaround”, Road & Track, November 1998, p. 201) high-rpm starting allowssubstantial elimination of the usual necessity of providing a fuel-richair/fuel mixture to start engine 40, reducing emission of unburned fueland improving fuel economy at start-up, particularly from cold.

More particularly, in conventional low-rpm starts, a rich mixturecomprising up to on the order of 6 to 7 times the stoichiometric amountof fuel is provided, to ensure that some fraction of the fuel is in thevapor phase, as only fuel in the vapor phase can be ignited by a spark.Most of the excess fuel condenses as liquid on the cold cylinder walls,and thus does not burn efficiently, if at all, and is immediatelyemitted unburned. By comparison, at high starting speeds according tothe invention, turbulence in the combustion chamber is sufficient toensure the presence of vapor, so that a near-stoichiometric mixture,typically including only 1.2 times the stoichiometric amount of fuel,can be provided to engine 40 during the starting phase. The avoidance ofrich mixtures at starting significantly reduces emission of unburnedfuel—since most of the fuel provided to a conventional engine atstarting is immediately exhausted unburnt—and provides some improvementin overall fuel efficiency.

Furthermore, as noted above, whenever possible—that is, whenever theengine is started except when immediate full torque is required by theoperator—a catalytic converter 64 is preheated to an effective workingtemperature of at least about 350° C. before starting the engine, toprevent even this relatively small emission of unburned fuel.

Thus, the primary consideration in selecting the torque of startingmotor 21 is that it be capable of rotating the engine 40 at about300-600 rpm for starting, and that it be capable of accepting at leastabout 30% of the engine's maximum torque output when operated as agenerator, so that the engine can be efficiently employed when chargingthe battery bank during extended low-speed operation; the mainconsideration in specification of the torque of engine 40 is that itprovides sufficient power for highway cruising while being operated athigh efficiency, i.e., that its maximum power output be sufficient tocruise in a range of desired cruising speeds; and the principalconsideration defining the power required of the traction motor 25 isthat it be sufficiently powerful to provide adequate acceleration incombination with the engine 40 and starting motor 21. Stateddifferently, the total power available provided by all of thesetorque-producing components should be at least equal to and preferablyexceeds the peak power provided by the internal combustion engines ofconventional vehicles of similar intended use, both as measured at thewheels. Moreover, as set forth in the '970 patent, the total torqueprovided by motors 21 and 25 should be at least equal to that producedby engine 40, in order to provide adequate low-speed performance undermotor alone, and without necessity of a variable-ratio transmission.

At the same time, motors 21 and 25 are also sized to be capable ofrecovering almost all of the vehicle's kinetic energy when operated asgenerators in the regenerative braking mode. A particularly highfraction of the vehicle's kinetic energy can be recovered duringlow-speed operation; as compared to high-speed operation, where airresistance and road friction consume a relatively large fraction of thetotal energy required, in low speed operation much energy is lost byconventional vehicles as heat released during braking.

Given the above considerations, the following are typical powerspecifications for the engine 40, starting motor 21 and traction motor25 of a 3000 pound vehicle having performance approximately equivalentto that of a “mid-size” sedan of United States manufacture. It should beunderstood that in these specifications, reference is made to the ratedpower produced continuously by the engine, not to the rated peak powerof the motors, as is generally conventional in the art. Further, themotors are specified assuming the direct-drive embodiment of FIG. 3; ifthe motors run at higher speeds, their ratings would be determinedaccordingly.

Engine 40: 40 to 50 horsepower at 6000 rpm

Starting motor 21: 10-15 horsepower at approximately 1500 rpm and higherspeeds

Traction motor 25: 50-75 horsepower from 1500 to 6000 rpm.

The same starting motor would be satisfactory for a larger, 4000 poundsedan, but the engine would typically provide 70-90 horsepower at 6000rpm and the traction motor 75-100 horsepower.

In both cases, the total power available from the electric motorstogether should equal, and preferably exceeds, the maximum poweravailable from the engine.

In the hybrid vehicle of the invention, which as noted does not requirea complex, heavy, and costly variable-ratio transmission, thesecomponents would provide acceleration much superior to that of typicalsimilarly-sized automobiles of United States manufacture, together withfar better fuel economy and substantially reduced emission ofpollutants. It will be apparent that these specifications may vary overrelatively wide ranges depending on the intended use of the vehicle ofthe invention, and should not be construed to limit the scope of theinvention.

As indicated above, in the preferred embodiment, both the starting andtraction motors are AC induction motors, although other types may alsobe employed. These motors, and the inverter/chargers controlling them inresponse to control signals from the microprocessor (as discussedfurther below), should be chosen and operated such that the motors havetorque output characteristics varying as a function of rpm asillustrated by curve A in FIG. 10. That is, the motors are operated bythe inverter/chargers, in response to control signals from themicroprocessor, so as produce constant torque up to a base speed C,typically 1500 rpm for a motor having a top speed of 6000 rpm, asemployed in the direct-drive embodiment of FIG. 3, and should produceconstant power at higher speeds; accordingly, the torque drops off atspeeds above the base speed C, as shown. The ratio of the base tomaximum speed, 4:1 in this example, can vary between about 3 to 1 andabout 6 to 1. This torque output characteristic essentially allows thevehicle of the invention to provide quite acceptable performance,especially acceleration, without the weight, complexity and cost of avariable-ratio transmission.

By comparison, the series-wound DC motors conventionally used asautomotive engine starting motors provide very high torque, but only atvery low speeds; their torque output drops precipitously at higherspeeds. Such conventional starter motors would be unsatisfactory in thepresent system.

FIG. 10 also shows the torque curve of a typical internal combustionengine at B; as noted, the torque is zero at zero rpm, so that a clutchallowing slippage is required to allow the engine to move the vehiclefrom rest. FIG. 10 shows at D typical curves for torque as measured atthe wheels of a vehicle propelled by a typical internal combustionengine driving the vehicle through a four-speed transmission, used toprovide additional torque at low speeds; the vertical spaces betweensections of curve D represent changes in gear ratio, that is, thevehicle will be shifted to move between the sections of curve D. Asshown by FIG. 10, the desired torque characteristics of the starting andtraction motors discussed above allow the vehicle of the invention toprovide low-speed performance comparable to or better than aconventional vehicle, while eliminating the necessity of avariable-ratio transmission. However, as discussed further below, it iswithin the invention of the present continuation-in-part application toextend the load-carrying capabilities of the hybrid vehicle of theinvention by also providing a variable-ratio, e.g., two-speed,transmission, where not excluded by the appended claims. This should notbe necessary with respect to passenger cars.

The ratio between the base speed and maximum speed of the motors as usedaccording to the invention is thus comparable to the ratio between thelowest and highest gears of a conventional transmission; for passengercars, the latter ratio is typically between 3 and 4:1, so that theengine's torque is relatively well matched to the road load over areasonable range of road speeds.

As discussed above, while it is within the scope of the invention tooperate the motors 21 and 25 and the internal combustion engine 40 atthe same maximum speed, so that no gearing is required to couple theseelements, it is presently preferred that at least traction motor 25 havea maximum speed substantially higher than that of the internalcombustion engine 40; the output shaft of motor 25 can be connected tothe road wheels by a chain-drive reduction unit, as indicated in FIG. 4.The maximum speed of the internal combustion engine is preferablylimited to on the order of 6000 rpm to limit wear, noise and vibration,which increase with higher operating speeds, and because engines capableof higher-rpm operation tend to have narrow ranges of rpm within whichthey produce substantial torque; the latter characteristic would beundesirable in a vehicle not having a variable-ratio transmission andintended to cruise powered solely by the internal combustion engine,according to the invention.

By comparison, operating the motors 21 and 25 at maximum speeds of9000-18,000 rpm allows them to be made smaller, lighter, and lesscostly; whether this advantage overcomes the added complexity of chain,gear, or belt drives, or other mechanical means allowing combination oftorque from the motors with that from the engine, is a matter ofengineering choice that may vary from one model of vehicle to the next.Both are accordingly within the present invention. If each of thetorque-producing components (that is, engine 40 and starting andtraction motors 21 and 25) is to be operated at the same speed, amaximum speed of approximately 6000 rpm is preferred, as this representsa good compromise between cost, weight, and size of the key components.

As discussed above, it is preferred that motors 21 and 25 have more thantwo poles, and be operated by current applied over more than threephases, so that failure of some components—such as the powersemiconductors used in the inverter/charger units, as discussedbelow—can be tolerated without total failure of the vehicle. It is alsodesired that the battery bank be divided into two, with the vehiclechassis connected between them, halving the voltage between givencomponents and the vehicle chassis, and thus simplifying theirconstruction, insulation, and connection. FIG. 5 shows a partialschematic diagram of a circuit providing these attributes.

The functions of the inverter/chargers 23 and 27 (separateinverter/chargers being required to allow independent operation ofmotors 21 and 25) include control of motors 21 and 25 to operate asmotors or as generators; operation of traction motor 25 in the oppositedirection for reversing the vehicle; conversion of DC stored by thebattery bank to AC for motor operation; and conversion of AC induced inthe motors when operated as generators to DC for battery charging.Essentially similar functions were provided by the solid-state switchingAC/DC converter 44 in the '970 patent; where not specified to thecontrary, the discussion thereof is applicable to the inverter designshown in FIG. 5 hereof.

As illustrated in FIG. 5, traction motor 25 is embodied as a five-phaseAC induction. motor; starting motor 21, which is not fully illustrated,as indicated, can be but is not necessarily generally similar. Othermotor types, such as permanent magnet brushless DC motors or synchronousmotors, might also be employed. The motors are operated as multiphasedevices, having three phases or more, permitting employment of smallerand overall less costly semiconductors, and allowing operation even ifsome of the semiconductors fail. Use of motors operated at relativelyhigh frequency, e.g., more than 60 Hz, also permits motors of a givenpower output to be smaller. As shown in FIG. 5, it is currentlypreferred that at least traction motor 25 be wired in the “wye”arrangement shown, rather than the known “delta” arrangement; it isfound that certain undesirable harmonics are reduced by the “wye”arrangement. Both are well known in the art, and within the scope of theinvention.

As illustrated in FIG. 5, each of the windings 78 of motor 25 isconnected to a pair of semiconductor switching elements 80 collectivelymaking up inverter/charger 27. Inverter/charger 27 is correspondinglyconfigured as a set of ten power semiconductors 80 controlled byswitching signals A through J provided by a pulse generator 88responsive to frequency, polarity and current signals received frommicroprocessor 48 (FIGS. 3 and 4). Typical operating frequencies can beup to 200, 400 or 600 Hz; the transfer of power between the battery bank22 and motors 21 and 25 is then controlled by pulse-width modulation,that is, by controlling the semiconductors 80 to conduct during portionsof the power waveform, the duration of the conducting portions varyingin accordance with the power required. Semiconductors 80 may be any typesuitable for handling relatively high voltages and currents;satisfactory insulated-gate bipolar transistors (IGBTs) are currentlyavailable and are presently preferred. As conventional, each of thesemiconductors 80 is paralleled by a freewheeling rectifier diode 82.

Design of the inverter/chargers 23 and 27 and of pulse generator 88 toprovide. suitable control signals A through T so that theinverter/chargers perform the functions listed above is within the skillof the art; again, see, for example, Bose, “Power Electronics andVariable Frequency Drives”, IEEE, 1996.

The current drawn from the battery bank 22 during long-term operation ofthe traction and starting motor(s) to propel the vehicle should belimited to 30-50 amperes, to reduce the size of the conductors and othercomponents required, as discussed in the '970 patent; these componentsare satisfactory to carry currents of up to 200 amperes, as may beencountered during full-power acceleration, as this condition will notpersist for more than about 30 seconds.

As indicated, the battery bank 22 comprises two substantially similarbattery assemblies 84; in one embodiment, each battery assembly willcomprise eight 48-volt batteries, such that 384 volts is provided byeach. The battery assemblies 84 are connected in series, so that 768volts are provided across the circuit “rails” 86, 88. However, thevehicle chassis connection is taken from between the series-connectedbattery assemblies, so that only 384 volts is present between any givencircuit component and the vehicle chassis; this “center-point-chassis”connection significantly reduces various insulation and heat-sinkingrequirements. More specifically, the conductors, connectors, relays,switches and like elements can be as approved by the National ElectricalManufacturers' Association (NEMA) for 600 volt service; such elementsare widely available, and are much more easily employed and much lessexpensive than those needed for continuously carrying current at, forexample, 300 volts and 300 amperes.

Preferably, as indicated by FIG. 5( a), illustrating a detail of aportion of one of the battery assemblies 84, the 48-volt batteries 85are connected by normally-open relays 87, so that the batteries 85 areisolated from one another under fail-safe conditions; for example, ifthe vehicle is involved in an accident, power to the relays is cut off,so that the maximum open voltage anywhere in the vehicle is 48 volts,reducing the danger of fire. Similarly, the relays open when thevehicle's “ignition” is shut off by the operator.

The present continuation-in-part application adds to the above from the'817 application that an auxiliary 12-volt system may also be provided,as shown at 223 in FIG. 14, discussed further below. This would be aDC-to-DC converter, allowing the vehicle to provide “jumping” current tostart other vehicles having conventional 12-volt electrical systems, andwould also allow the vehicle of the invention to be jumpstartedsimilarly, if necessary. Provision of a 12-volt system also allowsconvenient employment of conventional automotive accessories, such asradios and the like. The 12-volt system could perhaps most convenientlybe implemented by a separate semiconductor-implemented voltageconversion circuit, transforming the 48 volts from one of the batteriesto 12 volts for jumping others, and providing the inverse 12 to 48 volttransformation as needed. It should also be understood that theindividual batteries could be 42 volt units, conforming to the apparenttrend toward 42 volt systems for new vehicles. Further preferably, theentire battery bank assembly, including the relays, is enclosed in arugged container, significantly reducing the danger of electrical shockand the like.

Turning now to detailed discussion of the inventive control strategyaccording to which the hybrid vehicles of the invention are operated: asin the case of the hybrid vehicle system shown in the '970 patent, andas discussed in further detail below, the vehicle of the invention isoperated in different modes depending on the torque required, the stateof charge of the batteries, and other variables. Throughout, the objectis to operate the internal combustion engine only under circumstancesproviding a significant load, thus ensuring efficient operation. In thefollowing, the relationships between these modes are illustrated usingseveral different techniques, to ensure the reader's full understandingof various aspects of the vehicle control strategy; some of these areseen more clearly in one form of illustration than another.

FIG. 6 illustrates the several modes of vehicle operation with respectto the relationship between the vehicle's instantaneous torquerequirements or “road load”, the state of charge of the battery bank 22,and time, while FIG. 7 shows variation in, and the relationship between,road load, engine torque output, and the state of charge of the batterybank over time, that is, during an exemplary trip. FIGS. 8( a)-(d) showsimplified schematic diagrams of the vehicle of the invention in itsprincipal modes of operation, showing the flow of energy, in the form ofelectricity or combustible fuel, by dot-dash lines, and the flow oftorque by dashed lines. Finally, FIG. 9 provides a high-level flowchart,showing the principal decision points in the algorithm according towhich the microprocessor operates the various components of the hybridvehicle drivetrain according to the invention, and FIGS. 9 (a)-(c) showdetails and modifications thereof.

As noted, the preferred control strategy of the invention is illustratedin several different ways by FIGS. 6-9. The same specific numericalexamples for various significant control variables, data items, and thelike are used throughout for clarity. It will be understood that theseexamples would normally be expressed as ranges; although ranges are notused in the following, to simplify the discussion, it should beunderstood throughout that these numerical examples are exemplary only,and that the invention is not to be limited to the exact values of thecontrol variables mentioned herein.

Further, it should be realized that certain of these control variablesneed not be restricted to specific numbers; in some cases, the decisionpoints may be “fuzzy”, i.e., so-called “fuzzy logic” may be employed, sothat while the operating scheme retains its overall characteristics, thespecific values against which the control variables and data items aretested in implementation of the control strategy according to theinvention may vary from time to time. Examples of thispractice—amounting in many circumstances to modifying certain specificvalues depending on other data items not discussed in detail, or bymonitoring the vehicle's actual usage patterns over time—are givenbelow.

Given these several different explanations of the relationship betweenthe various operating modes of the vehicle of the invention, andspecifically these different illustrations of the combinations ofconditions in response to which the microprocessor controls modeselection, one of ordinary skill in the art would have no difficulty inimplementing the invention.

As noted, during low-speed operation, such as in city traffic, thevehicle is operated as a simple electric car, where all torque isprovided to road wheels 34 by traction motor 25 operating on electricalenergy supplied from battery bank 22. This is referred to as “mode I”operation (see FIG. 6), and is illustrated in FIG. 8( a). The same pathsof energy and torque may also be employed under emergency circumstances,referred to as mode III operation, as discussed below.

While operating at low speeds, e.g., when the vehicle's torquerequirements (“road load”, or “RL”) are less than 30% of the engine'smaximum torque output (“MTO”), engine 40 is run only as needed to chargebattery bank 22. starting motor 21 is first used to start engine 40, andis then operated as a generator by appropriate operation ofinverter/charger 23, so that charging current flows to battery bank 22.Accordingly, clutch 51 is disengaged, so that the road speed of thevehicle is independent of the speed of engine 40; engine 40 can thus beoperated at relatively high output torque level, for fuel efficiency.This “mode II” operation is illustrated in FIG. 8( b); as indicated,clutch 51 is disengaged, so that engine operation to charge battery bank22 through starting motor 21, and propulsion of the vehicle by tractionmotor 25, are completely independent of one another.

As in the '970 patent, engine 40 is sized so that its maximum torque issufficient to drive the vehicle in a range of desired cruising speeds;this requirement ensures that the engine is operated at high efficiencyduring normal highway cruising. Therefore, when a sensed increase in theroad load (e.g., by a continued operator request for more power)indicates that the preferred operating mode is changing from low-speedto highway cruising operation, the microprocessor controls startingmotor 21 by way of inverter/charger 23 to start engine 40. When engine40 is essentially up to speed, clutch 51 is engaged, so that engine 40drives road wheels 34 through the shafts of motors 21 and 25. When theoperator releases pressure on the accelerator pedal, indicating that adesired cruising speed has been reached, traction motor 25 isaccordingly depowered. The highway cruising mode is referred to as “modeIV” operation, and the flow of energy and torque are as illustrated inFIG. 8( c).

If extra torque is needed during highway cruising, e.g., foracceleration or hill-climbing, either or both of motors 21 and 25 can bepowered. This “mode V” operation is illustrated in FIG. 8( d); energyflows from tank 38 to engine 40, and from battery bank 22 to tractionmotor 25, and possibly also to starting motor 21; torque flows fromeither or both motors and engine to wheels 34.

The flow of energy during battery charging is not illustrated per se inFIG. 8, but will be understood by those of skill in the art, and isfurther described below. For example, when the engine's instantaneousoutput torque exceeds the road load, the starter motor 21 is operated asa charger, supplying recharging current to the battery bank. Similarly,when the road load is trending downwardly or is negative, either thetraction motor or the starter motor, or both, can be operated as aregenerative battery charger, supplying recharging current to thebattery bank; braking can be accomplished similarly in response to anappropriate operator command.

FIG. 6, as indicated above, is a diagram illustrating differing modes ofoperation of the hybrid vehicle powertrain of the invention; the modesof operation, indicated by numerals I-V, are plotted on a threedimensional chart, illustrating that the mode of vehicle operation ascontrolled by microprocessor 48 is a function of the state of charge ofthe battery bank, the instantaneous road load, and time. FIG. 7,discussed below, further illustrates the inventive mode of vehicleoperation.

FIG. 6 shows on one axis the state of battery charge extending from 70%at the origin outwardly to a minimum value shown of 30%. Normally thebatteries are maintained at least 30% of full charge. Preferably, thebattery bank is not charged to more than 70% of its theoretical fullcapacity; if a number of series-connected batteries were all charged to100% of their nominal full charge, some would likely be overcharged dueto manufacturing variation, local temperature variation and the like,which would significantly shorten their service life. Moreover,frequently recharging any individual battery to 100% of its theoreticalcapacity is deleterious to battery life as well.

The road load is shown in FIG. 6 on a second axis as varying from 0 atthe origin to 200% of the engine's maximum torque output. (Negative roadload, occurring during descents or under braking, is not shown in FIG. 6due to the difficulty of illustration. This circumstance is discussed inconnection with FIG. 7, below.) Time is shown on the third axisextending from an arbitrary point at the origin; that is, FIG. 6 showsthe mode of the vehicle's operation over the next short period of time(on the order of 30-60 seconds) from a present instant at the origin.Stated differently, according to one aspect of the invention, themicroprocessor 48 controls the vehicle's mode of operation at any giventime in dependence on “recent history,” as well as on the instantaneousroad load and battery charge state.

More specifically, FIG. 6 shows that during city driving (mode I),defined in this example as driving where the vehicle's instantaneoustorque requirements, or “road load”, is up to 30% of the engine'smaximum torque, the vehicle is operated as a “straight electric” car,the clutch being disengaged and energy from the battery bank 22 beingused to power traction motor 25 to propel the vehicle, as long as thebattery remains charged to between 50 and 70% of its full charge. If thecharge falls to below a given value, which may vary over time asindicated by the curved line defining the extent of mode II, mode II isentered as indicated, the engine is started, and the starter motor 21 isoperated as a generator to charge the battery to substantially fullcharge. As indicated in mode III, operation of the vehicle as anelectric car may also be permitted when the battery falls to below 40%of full charge, for example, if there is a fault in the engine orcharging system, but only on an emergency basis; such deep discharge isharmful to battery life.

During highway cruising, region IV, where the road load is between about30% and 100% of the engine's maximum torque output, the engine alone isused to propel the vehicle. Accordingly, when the microprocessor detectsthat transition between regions I and IV is required (e.g., themicroprocessor can effectively determine the road load by monitoring theresponse of the vehicle to the operator's command for more power), itcauses the starting motor 21 to spin the engine 40 to relatively highspeed; when a desired starting speed, typically 300 rpm, is reached, theelectronic engine management unit 55 and electronic fuel injection unit56 are controlled to fire the spark plugs and supply fuel, respectively,starting the engine. Thus starting the engine at relatively high rpmallows a near-stoichiometric fuel/air mixture to be used, as compared tothe much richer mixtures normally used for starting. Emissions ofunburned hydrocarbons are thus substantially reduced, and fuel economyimproved.

When the speed of the engine output shaft substantially matches that oftraction motor 25, clutch 51 is engaged; the power produced by motor 25is reduced as that produced by engine 40 is increased, so that thetransition between modes I and IV is smooth and essentially undetectedby the operator. When the operator reduces pressure on the acceleratorpedal 69, indicating that the desired cruising speed has been reached,power to motor 25 is reduced to zero.

If the operator then calls for additional power, e.g. for accelerationor passing, region V is entered; that is, when the microprocessordetects that the road load exceeds 100% of the engine's maximum torqueoutput, it controls inverter/charger 27 so that energy flows frombattery bank 22 to traction motor 25, providing torque propelling thevehicle in addition to that provided by engine 40. Starting motor 21 cansimilarly be controlled to provide propulsive torque.

As indicated above, during highway cruising, where the torque requiredto propel the vehicle varies as indicated by the operator's commands,the control system operates the engine at correspondingly varying torqueoutput levels. The range of permissible engine torque output levels isconstrained to the range in which the engine provides good fuelefficiency. Where the vehicle's instantaneous torque requirement exceedsthe engine's maximum efficient torque output, e.g., during passing orhill-climbing, one or both of the electric motors are energized toprovide additional torque; where the vehicle's torque requirements areless than the torque then being produced by the engine, e.g., duringcoasting, on downhills or during braking, the excess engine torque isused to charge the batteries. Regenerative charging may occursimultaneously, as torque from the engine and recovery of the vehicle'skinetic energy both drive one or both motors operated in generator mode.The rate of change of torque output by the engine may be controlled toreduce emissions, and in accordance with the state of charge of thebattery bank. FIG. 7 illustrates these relationships.

As mentioned above, FIG. 7, comprising FIGS. 7( a)-(c), and extendingover two sheets, is a timing diagram showing the relationship betweenroad load, engine torque output, the state of charge of the batterybank, and operation of the engine as these vary over time, duringlow-speed city driving, highway cruising, and extended high-loaddriving, thus further illustrating the control strategy employedaccording to the invention.

FIG. 7( a) shows the vehicle's instantaneous torque requirement, thatis, the “road load”, by a solid line, and the engine's instantaneousoutput torque by a dashed line, as these vary over time. (The engine'sinstantaneous output torque is repeated in FIG. 7( c), for clarity, andin order to clearly show certain additional aspects of the inventivecontrol strategy.) The road load is expressed as a function of theengine's maximum torque output. Where the road load exceeds the engine'sinstantaneous output torque, the cross-hatched areas between these twolines represent torque provided by the traction and or startingmotor(s); where the road load is less than the engine's instantaneousoutput torque, the cross-hatched areas represent charging of thebatteries.

It will be appreciated that positive vehicle torque demands correspondto steady-state cruising, acceleration, hill-climbing, or the like,while negative vehicle torque requirements correspond to deceleration ordescent. The engine's output torque is constrained to the range ofefficient operation; as illustrated in FIGS. 7 (a) and (c), this rangeis controlled to be between 30% and 100% of the engine's maximum torqueoutput (“MTO”). As mentioned above, it will be appreciated that the 30%figure, as well as similar figures mentioned herein, may vary withoutdeparture from the scope of the invention.

In the example of vehicle operation shown in FIG. 7, initially thevehicle is operated only at road loads below 30% of MTO, that is, intraffic, as indicated at A. Accordingly, all the torque required isprovided by the traction motor 25, and the state of charge of thebattery bank 22 (“BSC”), as illustrated by FIG. 7( b), correspondsdirectly to the road load; when the road load is negative, BSC increasesas the battery bank is charged by regenerative braking. (Changes in BSCare significantly exaggerated in order to clearly explain the eventsshown.)

At point B, the road load exceeds 30% of MTO for the first time on thisparticular trip. When this is detected by microprocessor 48, startingmotor 21 spins the engine 40 at relatively high speed, and the catalyticconverter 64 is preheated, causing a short drain on BSC, as shown at C.When the engine reaches the desired starting speed, e.g. 300 RPM, andthe catalyst reaches a minimum effective operating temperature, e.g. atleast about 350° C., the engine is started by supply of fuel and firingof its spark plugs, and the clutch is then engaged. As the engine isalready rotating at relatively high speed, and will have been warmed bycompression of air in its cylinders during the starting process, itbegins to produce useful torque almost immediately, as indicated at D.

Thereafter, when the vehicle's torque requirement exceeds theinstantaneous engine output torque, as at points E-G and P, one or bothof the traction and starting motors 25 and 21 are powered to provideadditional torque to the road wheels, that is, the vehicle is operatedin mode V. While the road load RL remains within the engine's efficientoperating range, e.g., while 30% MTO>RL>100% of MTO, the vehicle isoperated in mode IV. During mode IV operation, if the engine'sinstantaneous torque output exceeds the vehicle's torque requirement,but the battery is relatively fully charged, as at point H, the engine'storque output is reduced to match the road load; when MTO exceeds theroad load, and BSC falls below a predetermined level (see FIG. 7( b)),as at I and J, the excess torque available from engine 40 is used tocharge the batteries, as indicated at K and L (FIG. 7( c)). When thevehicle's torque requirement is less than the minimum permissible enginetorque output, as at M, the engine is again used to charge thebatteries, and regenerative braking is also performed, further chargingthe batteries. If the batteries become substantially fully charged,e.g., during a long descent, as at N, the engine may be shut offentirely, as seen at Q in FIG. 7( c).

More particularly, during deceleration or “coast-down”, the engine maybe “motored”, that is, driven by torque from the wheels, with the clutchengaged, but with at least the fuel supply shut off. In addition tousing no fuel, this has the advantage that when the operator nextrequires torque, e.g., when reaching the point at the bottom of a hill,the engine is rotating and can be immediately restarted by supply offuel. The exhaust valves might be opened during the motoring of theengine to reduce pumping losses.

The rate of change of the engine's torque output is limited, e.g., to 2%or less per revolution, as indicated by noting that the dashed line inFIG. 7( a), indicating the instantaneous engine output torque, lags thesolid line indicating the vehicle's instantaneous torque requirement.Thus limiting the rate of change of engine output torque is preferred tolimit undesirable emissions and improve fuel economy; that is, as thestoichiometric fuel/air ratio varies somewhat as the load changes,simply opening the throttle and causing additional fuel to be injected(as is typically practiced) upon the operator's depressing theaccelerator pedal would result in non-stoichiometric, inefficientcombustion. According to this aspect of the invention, the rate ofchange of engine torque is limited; this provides sufficient time forthe essentially conventional electronic engine management and electronicfuel injection systems, which comprise a “lambda sensor” 47 (FIG. 3) formonitoring the oxygen content of the exhaust gas stream as an indicationof stoichiometric combustion, to respond as the load changes, preservingstoichiometric combustion and reducing emission of unburned fuel.

The maximum permissible rate of change of engine output torque also maybe varied in accordance with the state of charge of the batteries; morespecifically, if the batteries are relatively discharged, it may bepreferable to allow the engine's output torque to ramp-up more quicklythan otherwise, in order to limit the amount of electrical power drawnfrom the batteries in response to an acceleration command. Moregenerally, it is preferred to operate the engine so as to limit theamount of power drawn from the batteries, as there are unavoidablelosses attendant on conversion of energy stored in the batteries tomotor output torque, and during the corresponding recharging period.

As mentioned above, FIG. 9 is a high-level flowchart of the principaldecision points in the control program used to control the mode ofvehicle operation. Broadly speaking, the microprocessor tests sensed andcalculated values for system variables, such as the vehicle'sinstantaneous torque requirement, i.e., the “road load” RL, the engine'sinstantaneous torque output ITO, both being expressed as a percentage ofthe engine's maximum torque output MTO, and the state of charge of thebattery bank BSC, expressed as a percentage of its full charge, againstsetpoints, and uses the results of the comparisons to control the modeof vehicle operation.

As noted above, certain control decisions involved in the inventivecontrol strategy illustrated in FIG. 9, and described therein as beingdetermined in response to precise criteria (in order to clearly presentthe main features of the inventive operating strategy), may instead beusefully somewhat “fuzzy”; in the present application, this term isintended to indicate that the value of a setpoint (for example) may varysomewhat in response to recent history, or in response to monitoredvariables not discussed above. As mentioned above, it is also to beunderstood that the values given above for various numerical quantitiesmay vary somewhat without departing from the invention. Specificalternatives are provided below for steps set forth in FIG. 9 thatimplement certain of these alternatives.

For example, in the example of the inventive control strategy discussedabove, it is repeatedly stated that the transition from low-speedoperation to highway cruising occurs when road load is equal to 30% ofMTO. This setpoint, referred to in the appended claims as “SP”, andsometimes hereinafter as the transition point (i.e., between operationin modes I and IV) is obviously arbitrary and can vary substantially,e.g., between 30-50% of MTO, within the scope of the invention.

It is also within the scope of the invention for the microprocessor tomonitor the vehicle's operation over a period of days or weeks and resetthis important setpoint in response to a repetitive driving pattern. Forexample, suppose the operator drives the same route from a congestedsuburban development to a workplace about the same time every morning;typically the road load might remain under 20% of MTO for the first fewminutes of each day, then vary between 0 and 50% of MTO for another fewminutes as the operator passes through a few traffic lights, and thensuddenly increase to 150% of MTO as the operator accelerates onto ahighway. It is within the skill of the art to program a microprocessorto record and analyze such daily patterns, and to adapt the controlstrategy accordingly. For example, in response to recognition of aregular pattern as above, the transition point might be adjusted to 60%of MTO; this would prevent repetitive engine starts as the road loadexceeded 30% of MTO for a few hundred yards at a time, as might oftenoccur in. suburban traffic. similarly, the engine starting routine mightbe initiated after the same total distance had been covered each day.

It is also within the scope of the invention to make the setpoint SP towhich the road load is compared to control the transition from mode I tomode IV somewhat “fuzzy”, so that SP may vary from one comparison ofroad load to MTO to the next depending on other variables. For example,as discussed above, if during low-speed operation the operator depressesthe accelerator pedal rapidly, this can be treated as an indication thatfull power will shortly be required, and the engine-starting operationbegun before the road load reaches any particular setpoint SP.

The value of the transition point may also vary in dependence on themode of operation in effect when the road load equals a given setpointSP. For example, suppose the setpoint at which the mode of operation iscontrolled to change from the low-speed mode to the highway cruisingmode is normally set to 30% of MTO, as in the examples discussed above.If traffic conditions were such that the road load fluctuated aroundthis value, and engine operation were controlled solely in response toroad load, the engine would be repeatedly started and shut off as theroad load exceeded 30% of MTO for a few hundred yards at a time, andthen fell back below 30% of MTO, as might often occur in suburbantraffic. Repeated restarts might also occur if the road load averagedover 30% of MTO but occasionally dropped below this value, as mightoccur in moderate-speed, flat-road cruising.

By monitoring the road load over time, and comparing it to differentsetpoints accordingly, much of this undesirable repetitive sequence ofengine starting and shut-off can be eliminated. It might be preferableto commence mode IV operation upon the occurrence of differingconditions; for example, mode IV might be entered from mode I only afterthe road load exceeded a first, lower setpoint SP for an extended periodof time, so that the engine would be run for extended low-speedcruising, but to start the engine immediately if the road load exceededa higher setpoint SP2, e.g. 50% of MTO, as during acceleration tohighway speed. Similarly, the engine might preferably be shut down onlyif the road load was less than a minimum setpoint for mode IV operationfor an extended period of time. Thus providing “hysteresis” in themode-switching determination would limit repetitive engine starts incertain types of driving. These limits could be further adjusted as thedriving pattern became clear, i.e., as discerned by the microprocessor.

In a further refinement, the setpoint at which the engine is shut off asthe road load dropped below the usual minimum value for mode IVoperation could vary dependent on BSC; if the batteries weresubstantially fully charged, the engine might be shut off as road loaddropped below 30% of MTO, but if their charge was lower the engine mightbe controlled to continue to run, even at a stop, i.e., zero road load,to charge the batteries. Of course, the clutch would still have to bedisengaged at when the road load fell below 20-30% of MTO, in order thatthe engine could run at an efficient speed for production of torque.

FIG. 9 thus shows the main decision points of the control program run bythe microprocessor, with the transition point between mode I, low-speedoperation, and mode IV highway cruising, set at a road load equal to 30%of MTO. Examples are then given for some of the various optionsdiscussed above, by substituting various of the decision points withalternatives indicated below. Other optional points not specificallyshown but discussed herein are within the scope of the invention.

The control program is entered at step 100, where the microprocessordetermines whether the road load RL is less than 30% of MTO. If theanswer is yes (“Y”), the clutch is disengaged if necessary as indicatedat steps 103 and 105. The state of charge of the battery bank BSC isthen tested at step 110; if BSC is between 50 and 70% of full charge,the vehicle can operate for some time as a straight electric vehicle,and mode I is accordingly entered, as indicated at 115. A “mode I” loopis then established, including steps 100, 103, and 110; as long as allconditions tested in these steps remain stable, the vehicle continues tobe operated in mode I.

However, if at step 110 it was determined that BSC was less than 50% ofits maximum value (“N”), the engine should be run, if possible, tocharge the battery bank, up to, for example, 75% of its maximum charge,as tested at step 120. If the engine is already running, as tested atstep 125, the battery is charged as indicated at 130, and a stable “modeII” loop, as noted at 135, is established including steps 100, 103, 110,120, 125, and 130. (Normal operation of step 110 would be bypassed ordisabled in this mode to prevent battery charging from being stoppedwhen BSC reaches 70%). If the engine is not running, an engine startingsubroutine (shown separately, by FIG. 9( a), is entered, as indicated atstep 140.

In the engine starting subroutine, beginning with the ‘enter’ block 141,the clutch is disengaged if necessary at steps 142-143, and the catalysttemperature is tested at 145, to determine whether it is at least about350° C.; the catalyst is heated as necessary, as indicated at 150. Whenthe catalyst is heated suitably, the engine is then spun by the startermotor until a desired starting speed is reached, as indicated by theloop including blocks 155 and 160. When the engine reaches its desiredstarting speed, it is started at step 165, by supply of fuel and firingof its spark plugs, concluding the engine starting subroutine asindicated by ‘return’ block 170. If the engine starting subroutine wasentered from the mode II loop, as above, the battery bank may then becharged as indicated at 130.

If in performance of step 120 it appeared that BSC was less than 40%,which would only occur upon failure of the engine or charging system,step 175 may be performed; thus, if 30%<BSC<40%, the vehicle may beoperated in mode III as an electric car, to provide emergency operation.However, this should be strictly limited to avoid deep discharge of thebattery bank, tending to shorten its useful life. As indicated at 177,the vehicle is completely disabled if BSC falls below 30%.

If RL is determined to exceed 30% of MTO in step 100, the program goesto step 180, where the term 30%>RL>100% is evaluated; that is, themicroprocessor determines whether the road load is appropriate forhighway cruising in mode IV. If so, and if the engine is running, astested at step 190, a stable loop including steps 180 and 190 isestablished; the system remains in mode IV, as indicated at 185, untilthe state of one of these tests changes.

If in step 190 it is determined that the engine is not running, theengine start subroutine, starting with step 140 as discussed above, isentered as indicated at 195; upon return, at 200, the clutch is engagedat 210, and the loop including steps 180 and 190 is entered.

As noted, in step 180 it is determined whether RL is between 30 and 100%of MTO; if not, it is determined in step 220 whether RL is greater than100% of MTO. If so, mode V is entered, and the traction motor (andoptionally the starting motor) is powered to provide additional torquepropelling the vehicle, as indicated at 230. A loop including steps 220and 230 is thus established, so that mode V remains stable until thestate of the test performed in step 220 changes.

When in performance of step 220, it appears that RL is now less than100% of MTO, it is then determined in step 215 whether RL is less than30% of MTO. If so, the engine is shut off, as indicated at 240, and theprogram returns to step 100; if not, the program is returned to step180.

It will be appreciated that according to the FIG. 9 flowchart, it ispossible for the system to proceed directly from mode I to mode V, thatis, from step 100 to step 220, if the road load rapidly increases fromless than 30% of MTO to more than 100% of MTO. Permitting the operatorto thus operate the system is an important safety feature, for examplewhen fast acceleration from a stop is required to merge into highwaytraffic. In these circumstances the engine would not be running duringinitial operation in mode V, necessitating a significant drain on thebattery bank and overdriving the traction motor. Accordingly, stepsequivalent to steps 190, 195, and 210 (including the engine startingsubroutine) are to be understood to follow step 220 and precede step230. That is, in the event mode IV was effectively omitted in passingdirectly from mode I to mode V, the engine is started and the clutchengaged as soon as possible; these duplicate steps are not shown, forclarity.

In the above discussion of FIG. 9, it was assumed that the transitionpoint between low-speed and highway operation is set so that thetransition occurs when the road load is equal to 30% of MTO under allcircumstances. However, as discussed above, it may be desirable tooperate the system so that the vehicle goes from the low-speed mode I tothe highway-cruising mode IV at a higher road load, e.g., 50% of MTO,than the road load at which the low-speed mode is reentered, e.g., whenroad load in mode IV falls to below 20%. This “hysteresis” of the modeswitching point—for example, allowing the vehicle to accelerate in modeI up to road loads of up to 50% of MTO, but not shutting the engine off,ending mode IV operation, until road load falls below 20% of MTO—avoidsexcessive mode-switching during periods of fluctuating road load.

For example, in typical. suburban traffic, one might commonly acceleratepast 30% of MTO, to what might otherwise be a normal cruising speed, butstop again shortly thereafter; it would be inefficient to thusrepetitively stop and restart the engine as the load fluctuates around30%. Hysteresis might similarly be useful in avoiding needless modeswitching in moderate-speed, flat road cruising in mode IV, when theroad load might well occasionally drop below 30%; again, it would beinefficient to repeatedly shut off and restart the engine.

Thus providing differing mode switching points depending on thedirection of the change in road load can be accomplished readily bymonitoring the road load RL as a function of time, and takingappropriate control action. For example, if the system is maintained inmode I until RL exceeds the “normal” 30% of MTO mode switching point fora period of, for example, 30 seconds, and without exceeding 50% of MTO,the excessive mode switching otherwise likely to be encountered insuburban traffic can be largely avoided. FIG. 9( b) shows a step 100′replacing step 100 in FIG. 9 and implementing this “low-speedhysteresis”. As indicated, the system remains in the low-speed mode I aslong as RL is less than 30% of MTO, or unless RL exceeds 30% of MTO formore than 30 seconds, or exceeds 50% of MTO; if either of the latterconditions occurs, the program goes to step 180, initiating mode IVoperation.

Similarly, hysteresis in mode IV cruising, in order to implementexcessive mode shifting that might otherwise occur if the road loadfluctuates around a fixed mode switching point, can be implemented bysimply providing that the system remains in mode IV as long as RLremains between 30 and 100% of MTO, unless RL is less than RL for morethan 30 seconds, or exceeds 100% of MTO. This can be implemented asshown in FIG. 9( c); a revised step 215′ replaces step 215 of FIG. 9,and provides that, if the system is in mode IV, unless RL is less than30% of MTO for more than 30 seconds, step 180 is re-entered, thuspreserving the “mode IV loop”; when RL is less than 30% of MTO for morethan 30 seconds, the engine is shut down, at step 240, control is passedto step 100, and mode I re-entered.

Numerous further modifications to the detailed control strategy of theinvention as illustrated in FIGS. 6-9 will occur to those of skill inthe art, and are within the scope of the invention. For example, it maybe desirable to vary the operation of the system insofar as responsiveto BSC in accordance with monitored variables indicative of batterytemperature, ambient temperature, and the like; e.g., on a hot day itmay be advisable to avoid charging the battery bank to more. than 60% offull charge, as this may cause overheating. Further, as noted above thetransition points between modes I, IV, and V in particular may vary inaccordance with the operator's commands, so as to provide maximumvehicle responsiveness for safety and ease of consumer acceptance, andover periods of days or weeks, as the microprocessor builds up adetailed historical record of the vehicle's usage pattern, from which anoptimized control strategy may be derived.

It may also be possible to provide the microprocessor with usefulcontrol information from the operator, without requiring the operator tounderstand the workings of the system in detail. For example, operatorsare now well-accustomed to set a “cruise control” when a desiredcruising speed is reached; thereafter, existing engine managementsystems control the instantaneous engine torque output with respect tovariation in the road load to maintain vehicle speed substantiallyconstant. It would be a to simple matter for the microprocessor toaccept a desired cruising speed thus input by the operator, as indicatedin FIG. 4. The operator would then be relieved of continuous throttlecontrol, and the microprocessor would similarly control theinstantaneous engine torque output with respect to variation in the roadload to maintain vehicle speed substantially constant, both asconventional; however, according to the invention, the microprocessorwould also reset the transition point so that the system would remain incruising mode IV until the operator had indicated to the contrary, i.e.,by exiting cruise mode.

As discussed above, according to a further embodiment of the invention,additional flexibility is provided to the hybrid vehicle as describedabove by providing a turbocharger 100, also controlled by themicroprocessor 48, so as to be operated when useful in further improvingvehicle efficiency and drivability and not at other times. Providing the“turbocharger-on-demand” allows the engine to function efficiently indifferent torque output ranges, as needed. Essentially, the turbocharger100 is employed only when the vehicle's torque requirements, the “roadload” as above, exceeds the engine's normally-aspirated maximum torquecapacity for a relatively extended period T of time, for example, duringextended high-speed driving, towing a trailer, or driving up a longhill. Where the road load exceeds the engine's maximum torque for arelatively short period less than T, the traction motor (and possiblyalso the starting motor) are used to provide additional torque, as inthe '970 patent and above. According to a further aspect of theinvention, the period T is controlled in response to the state of chargeof the battery bank; when the battery bank is relatively depleted, theturbocharger is activated sooner than otherwise, so as to preserve thebattery bank.

As is well known to those of skill in the art, a turbocharger 100 (seeFIG. 11) typically comprises two turbine wheels 102 and 104 on a commonshaft 106, referred to herein as the exhaust-side and air-side wheelsrespectively. The flow of exhaust gas from engine 40 causes exhaust-sidewheel 102 to spin; air-side wheel 104 is driven by shaft 106, drawingair into the body of turbocharger 100 through air filter 110. Waste heatin the exhaust stream is thus effectively recovered by compressing theintake air, which is then ducted to the intake manifold 122 of engine40. Additional fuel can be burned in the additional air thus provided,so that additional torque is produced. The compressed air may be cooledadiabatically by heat exchange with ambient air in intercooler 117 ifdesired, further improving thermal efficiency of engine 40.

In typical turbocharger operation, a “wastegate” 114 is provided tolimit the exhaust pressure incident on exhaust-side wheel 102, thuslimiting the speed of air-side wheel 104 and regulating the “boost”provided by the turbocharger. The waste gate may be spring-loaded toopen at a fixed boost pressure (as typically provided to regulate theoutput of turbocharged racing engines) or may be controlled in afeedback loop using the pressure in the engine intake manifold as thecontrol variable. See Automotive Handbook, 2nd Ed., Robert Bosch GmbH(1986), p. 356.

Further, in conventional practice, the turbocharger is used at alltimes, and the engine's design is optimized accordingly. For example,turbocharged gasoline engines typically have compression ratios of 7 or8 to 1, as compared to 9-11 to 1 for normally-aspirated engines. Neitherpractice is employed according to the present invention; theturbocharger is controlled by the microprocessor to operate only whenneeded, and the engine's compression ratio, and other design parameters,are selected based on design criteria relevant when operated in thenormally-aspirated mode.

According to the present invention, the waste gate 114 is controlled bythe microprocessor 48; except under circumstances when the extra powerprovided by turbocharging is needed, the waste gate 114 is open (asshown in FIG. 1), so that the engine exhaust essentially bypasses theturbocharger 100. A valve 120, also controlled by microprocessor 48, mayalso be provided in the duct connecting the air side of the turbocharger100 and the intake manifold 122 of the engine, so that the engine 40draws air through the turbocharger only when in use; a second air filter124 is then also provided.

Commonly, turbocharging for automotive use is employed in order thatrelatively small-displacement engines will produce high horsepower atthe upper end of their operating range; the other design parameters ofsuch engines (e.g., camshaft profiles) are chosen similarly. Enginesthus optimized for high-rpm horsepower produce reduced low-speed torque,that is, are “peaky” compared to normally-aspirated engines. Avariable-ratio transmission is essential to obtain reasonableacceleration from low speeds. Stated differently, turbocharging asusually implemented for automotive use provides relatively high torqueat the upper end of the engine's speed range, but relatively poor torqueat lower speeds; such an engine would be unsuitable in practice of thepresent invention. Moreover, turbocharged engines typically suffer“turbo lag”, that is, slow response to sudden increase in torquerequired. As discussed further below, this particular problem isovercome by use of the turbocharger in a hybrid vehicle according to theinvention.

Those of skill in the art will recognize that turbocharged engines arealso used in heavy-load road vehicle applications, such as trucks andthe like, but these vehicles demand transmissions having 12, 16, or moreratios, so that the engine's narrow power peak can be matched to theload, and exhibit extremely poor acceleration, as well as excessivegear-changing and cost, all of which would be unacceptable to theordinary motorist. Thus, normally-turbocharged engines, of both thelow-speed truck type, or the high-speed automotive type, are notsatisfactory in implementation of the present invention.

As also noted above, as conventionally employed, a turbocharger is usedat all times. By comparison, according to the present invention, theturbocharger is controlled by the microprocessor 48 to be used onlyunder specified driving conditions, allowing the engine to be operatedefficiently in other modes.

FIG. 12, as indicated above, is a diagram comparable to FIG. 6. Thediffering modes of operation of the hybrid vehicle powertrain of theinvention shown thereon are identical to those of the FIGS. 3 and 4vehicle illustrated in FIG. 6, with the addition of turbocharged modeVI. Similarly, FIG. 13 is similar to FIG. 7, but illustrates theoperation of a vehicle including a “turbocharger-on-demand” according tothis aspect of the invention.

As shown in FIG. 12, according to this aspect of the present invention,a further region VI is provided, wherein the turbocharger 100 isactivated by the microprocessor 48 when it detects that the road loadhas exceeded the engine's maximum output for more than a period of timeT. Typically these events will occur when the vehicle is towing atrailer or is otherwise heavily laden, is climbing a long hill, or isoperated at high speed for a long period of time.

More specifically, when the road load only exceeds the engine's maximumpower for a short time, less than T, as during acceleration onto ahighway or during passing, the traction motor is employed to provide theadditional torque required, as described above. When the road loadexceeds the engine's maximum power for a time greater than T, theturbocharger is energized by closing waste gate 114, and operating valve120, if provided, to open the duct between the air-side of turbocharger100 and the intake manifold 122 of engine 40. As the turbocharger“spools up” to its operating speed range, the maximum torque produced byengine 40 increases, and the torque produced. by traction motor 25 isgradually reduced. This sequence of events is discussed further below inconnection with FIG. 13.

FIG. 12 also shows, by the angle of the line separating regions V and VIwith respect to the t=0 plane, that T can vary with the state of chargeof the battery bank 22; when the battery bank is fully charged, T islonger—that is, energy from the battery bank is used to satisfy roadload in excess of the engine's maximum torque output for a longerperiod—than when the battery bank is relatively less fully charged. Theturbocharger can also be operated to provide additional engine powerwhen full acceleration is needed, e.g., upon detection of the operator'saggressively pressing the accelerator pedal down completely.

As mentioned above, FIG. 13, comprising FIGS. 13( a)-(c), and extendingover two sheets, is a timing diagram showing the relationship betweenroad load, engine torque output, the state of charge of the batterybank, and operation of the engine in electric car, normally-aspiratedand turbocharged modes as these vary over time, during low-speed citydriving, highway cruising, and extended high-load driving, thus furtherillustrating the control strategy employed according to the invention.FIG. 13 is essentially identical to FIG. 7, with the addition ofillustration of the operation of turbocharger 100 when the road loadexceeds 100% of MTO for more than a period of time T.

Thus, as shown in FIG. 13( a) at t₁, t₂, t₃, and t₄, the microprocessormonitors the length of time t during which road load exceeds 100% ofMTO, and compares t continually to a value T preferably varied inaccordance with BSC; this is shown by the relative lengths of the arrowsmarked T on FIG. 13( b). While t<T, as at E, F, and G in FIG. 13( a),the excess torque required by the road load is provided by either orboth of the traction and starting motors, drawing power from the batterybank. Note that the motors together are rated to be capable ofcontinuously providing torque up to at least 100% of MTO, in accordancewith the '970 patent; this allows the motors to provide adequate torquefor good vehicle performance without a variable-ratio transmission. Themotors may also be overdriven to provide more than their rated torque,well over 100% of MTO, for short periods of time, t<T, as at F; asnoted, according to an important aspect of the invention, where torquein excess of MTO is needed for a longer period of time, t>T, theturbocharger is activated.

Thus, when t₄≧T, as at P, the microprocessor activates the turbochargeressentially as discussed above, that is, by closing waste gate 114 andvalve 120 (if provided). As the turbocharger “spools up”, which may takesome seconds, and the boost it provides increases, as indicated at Q,the torque provided by the traction motor (and possibly also by thestarting motor) is decreased accordingly, as indicated at R. Theoperator need not be aware of or take any action to initiate theturbocharger's activation; this is controlled by the microprocessor inresponse to monitoring the road load over time and the state of chargeof the battery bank.

As discussed in connection with both FIGS. 12 and 13, T is preferablyvaried in accordance with BSC, so that the turbocharger is activatedrelatively sooner when BSC is relatively low; this limits the amount ofenergy drained from the battery during operation of the engine and thetraction motor (or both motors) when the road load exceeds 100% of MTO,so that BSC does not fall to an undesirably low value.

Those of skill in the art will recognize that provision of amicroprocessor-controlled turbocharger in a hybrid vehicle according tothe invention permits operation in an additional mode, providingincreased flexibility in the operational scheme provided; essentiallythe turbocharger provides a larger engine only when needed, at no costin efficiency at other times. This is particularly significant inmeeting the goals of the hybrid vehicle of the invention. Morespecifically, in addition to the operational advantages noted, provisionof a “turbocharger-on-demand” in the hybrid vehicle according to theinvention allows the engine to be smaller than otherwise, that is, toprovide adequate highway performance in a vehicle of a given weight. Asthe starting motor/generator must be sized such that when it is operatedto charge the batteries (e.g., in extended city driving) it loads theengine adequately that the engine is operated efficiently, employment ofa smaller engine allows use of a smaller generator motor. For similarreasons, provision of a smaller engine allows it to be used toefficiently propel the vehicle in highway driving commencing at loweraverage speeds, resulting in turn in better fuel economy. By providingthe “turbocharger-on-demand” according to the invention, all theseadvantages can be realized without sacrifice in the ultimate performanceof the vehicle.

As noted above, one convenient implementation of the“turbocharger-on-demand” according to the invention is to operate thewastegate by a solenoid or the like controlled by the microprocessor,that is, to employ the wastegate as a bypass valve except whenturbocharged operations are desired. A separate bypass valve might alsoor alternatively be provided. The wastegate is still preferablyimplemented as a spring-loaded relief valve, as illustrated in FIG. 11,and as generally conventional, to limit the “boost” provided. It is alsowithin the invention to operate the waste gate to take intermediatepositions, that is, between fully-open and closed positions, so as tolimit the torque to limit wheelspin as detected, and to keep theturbocharger wheels spinning at an intermediate speed, to reduce thetime necessary to “spool up” to full speed. It is also within theinvention to adjust the wastegate responsive to an atmospheric-pressuresignal provided by a suitable sensor 107 (FIG. 11) to ensure thatadequate boost is provided at higher altitudes to ensure vehicleperformance.

It will also be appreciated that a supercharger, that is, apositive-displacement air pump driven by the engine, could be used toimplement the differing modes of vehicle operation illustrated in FIGS.12 and 13; for example, the supercharger's operation could be controlledby the microprocessor by driving it through an electrically-controlledclutch, and this is accordingly within the invention. However, thiswould be less efficient than turbocharger operation, as turbochargingeffectively recovers some of the waste heat in the engine exhaust bycompressing the air reaching the inlet manifold, while superchargingconsumes engine torque. Turbocharging, as discussed in detail, isaccordingly preferred.

It will therefore be appreciated that by providing theinternal-combustion engine of a hybrid vehicle with a turbochargercontrolled by the vehicle's controller to operate only during extendedperiods of high torque requirements, a number of important advantagesare realized, both as compared to a conventional system wherein theturbocharger is continually activated, or as compared to a large enginehaving the same maximum torque as the smaller turbocharged engine. As tothe latter, as explained above all internal combustion engines areextremely inefficient, except when operated at near peak torque output;the larger the engine, the less frequently this will occur. As to theformer, employing a conventionally-turbocharged engine, having thetypical “peaky” torque curve, would not allow the engine to be used topropel the vehicle during highway driving without a variable-speedtransmission. Instead, by providing a “turbocharger-on-demand”, that is,which is only employed when it is actually needed, the vehicle of theinvention can employ a small engine optimized for its main function ofpropelling the vehicle efficiently during highway cruising, and which isoperable as a much larger engine when needed.

Other advantages provided by the invention include the fact that as thewastegate is. normally open, the exhaust temperature will stay high,optimizing catalytic converter performance; as conventionallyimplemented, cooling of the exhaust gases as their energy is removed inspinning the turbocharger rotor can prevent good catalytic converterperformance, especially at low speeds. Further, because the tractionmotor provides additional torque when needed, the “turbo lag”experienced in conventional turbocharged vehicles as the turbocharger“spools up” when the operator calls for more power is eliminated.

When constructed and operated according to the invention, that is, as ahybrid vehicle having an internal-combustion engine with a turbochargercontrolled by the vehicle's controller to operate only during extendedperiods of high torque requirements, even a heavy vehicle having pooraerodynamic characteristics, such as a sport-utility vehicle or van, canoffer good acceleration and hill-climbing and towing ability, whilestill providing extremely good fuel economy and extremely low emissions.

Another aspect of the invention concerns the method of sizing thevarious components of the system. Examples were given above of componentselection for a vehicle not including a turbocharger according to thisaspect of the present invention. Using as a further example a 5,500pound “sport-utility vehicle” (“SUV”) required to have reasonableacceleration and passing performance even while towing a 6,000 poundtrailer, sizing of the components of the hybrid drive system of thepresent invention is preferably accomplished as follows:

1. An internal combustion engine is selected which has sufficient torqueto drive the SUV without trailer at medium to high speed along amoderate grade. More specifically, a typical specification will requirethat the engine be sufficiently powerful to proceed up a 6% grade ofunlimited extent at 50 mph. An engine of 100 hp at 6,000 maximum RPM isappropriate to meet this requirement for the SUV described above.

2. If a trailer is to be towed, a turbocharger, operated as above, isadded. The turbocharger is sized so that when it is operated the engineprovides up to 140 hp.

3. The charger motor is sized so as to provide an engine load equal toapproximately 70% of the engine's maximum torque at a suitable enginespeed. In this way fuel is used efficiently during battery charging, asdiscussed above. In the example, the charger motor is preferably aninduction motor of 15-30 hp capacity, possibly configured as a“faceplate” or “pancake” type, essentially forming the flywheel of theengine. Such a motor can be operated as a generator requiring 20-22 hp,which is 70% of the maximum torque produced by the engine specifiedabove when operated at 1200-1500 rpm; battery charging can thus beaccomplished in a very fuel-efficient manner. This is essentiallyequivalent to specifying the starter/generator based on its ability toaccept at least about 30% of the engine's maximum torque output (MTO, asabove); in this way the engine is operated at a fuel-efficient powerlevel during charging.

4. The traction motor is sized to provide adequate torque at zero speedto overcome the maximum grade specified from rest, with the startermotor assisting as needed. In the example the traction motor may be aninduction motor of 100 hp, with a maximum speed of 16,000 rpm, and beconnected to the drive wheels through a chain drive providing theappropriate reduction ratio. It will be appreciated that in this examplethe total torque available from the starting and traction motorscombined exceeds that provided by the engine, in accordance with anaspect of the invention of the '970 patent.

5. The torque vs. speed profile of the traction motor is selected toallow city driving, in particular, to provide acceleration sufficient toconform to the Federal urban driving fuel mileage test (“FUDS”), withoutuse of torque from the engine.

6. The battery capacity is then selected to provide sufficient cyclelife, i.e., so as not to be overstressed by deep discharge over manyrepetitive driving cycles. In the example, an 800 v, 8.5 KAH batterypack is provided. The battery bank should be sized and arranged so thatthe maximum current to be absorbed with the starter/generator beingdriven at 30% of MTO is no more than 50 amperes.

7. Finally, the controller is provided with software to implement thecontrol scheme described in detail above, that is, to use the tractionmotor as the only source of drive torque at low speed, to start theengine when the road load increases beyond a setpoint, to operate theturbocharger when the road load exceeds the engine's maximum torque formore than a prescribed time T, which may be varied in accordance withthe state of charge of the batteries, and otherwise as described above.Essentially, the controller is operated so that the engine is onlyoperated in a fuel-efficient range, e.g., driving a load at least equalto 30% of MTO.

Simulations show that vehicles configured as above will generally becapable of 80-100% improvement in fuel economy with respect toconventional vehicles of similar size, weight and performancecharacteristics.

Further Improvements According to the Continuation-in-Part ComponentSpecification

In addition to the methods of sizing the components of the powertrainand ancillary components set forth above, another method of doing so isgenerally as follows. As set forth above, it is desirable for a numberof reasons to operate the system of the invention at relatively highvoltages, e.g., 800 V or above, in the case of larger vehicles; thisreduces the current flowing throughout the system, which allows use ofplug-in rather than bolted connectors, allows use of inexpensiveautomatic disconnects, and reduces resistance heating losses.

More particularly, suppose that the “average maximum” current (e.g.,defined as the maximum current flowing for more than, for example,thirty seconds; under. most circumstances, the average current would bemuch less) is controlled to be 50 A. This allows use of inexpensivemass-produced plug-in connectors, and can be controlled by inexpensivemass-produced power electronic components, as needed to construct theinverter/charger units. These components can be designed to conduct upto approximately 200 A for up to thirty seconds, so that fullacceleration can be provided for a time sufficient for the vehicle toreach essentially its maximum speed; according to this aspect of theinvention, the peak current can accordingly be set at, for example, 150A, and the power electronics components then sized based on this value.

More particularly, it appears useful to size the components with respectto one another, in particular, the battery bank with respect to thetraction motor(s), so that the peak current is no more than about 150 A,and so that under peak electrical loading (usually under acceleration) aratio of at least 2.5:1 of the battery voltage to the peak current isexceeded.

For example, suppose it is desired to implement the invention withrespect to a relatively heavy, e.g., 6000 pound, vehicle having targetacceleration capabilities such that a 120 HP electric traction motor,typically drawing 100 kW, will be required. The battery bank for such avehicle is sized to provide a nominal voltage of 830 V (i.e., when notunder load); this will drop to approximately 650 V under load. Thebattery bank will thus be required to produce 153 A (=100 kW/650 V)during full acceleration, and the ratio of voltage to peak current is3.92 (=650 V/153 A).

In another example, of a much lighter 3000 lb vehicle, a 80 HP, 60 kWmotor might be sufficient. To keep the peak current to 115 A, a batterybank of 600 V nominal, 500 V under load would be required. The ratio isthen 4.3 (=500V/115 A).

By comparison, insofar as known to the inventors, the Toyota “Prius”hybrid car now being marketed uses a 30 kW motor, and its battery bankprovides approximately 230 V under load; the current required is thusapproximately 120 A (=30 kW/230 V) and the ratio between the voltageunder load and the peak current is only about 2 (=230V/120 A). The motorin the Prius is incapable of providing adequate acceleration withoutassistance; this in turn requires that an internal combustion engine(ICE) be provided, and be connected to the wheels by way of avariable-ratio plantary gearset. Operation of the ICE in the Prius isthus constrained by the vehicle's torque requirements, whichunacceptably complicates its operation and renders it incapable ofmaximally efficient operation.

Applicants assert, therefore, that according to the invention thecomponents of the hybrid vehicles of the invention are to be sized sothat the ratio between battery voltage under load to peak current is atleast about 2.5, and preferably is at least 3.5 to 4:1; this allowsadequate acceleration from low speeds without use of torque from theICE, which in turn allows elimination of any multiple-speed orvariable-ratio transmission, and allows the ICE to be declutched fromthe wheels except when the ICE can be employed efficiently to propel thevehicle (or the ICE is being motored during deceleration or coast-down,as above). In turn this requirement leads to operation at highervoltages than typical, to keep both average maximum and peak currentslow, which provides the very significant advantages mentioned above.

Range-Broadening Transmission

As mentioned above, in some embodiments of the invention as disclosed bythe present continuation-in-part application, a two-speed transmissionmay be provided to broaden the range of utility of the vehicle. Anexemplary hybrid vehicle powertrain providing this and furtheradditional features is shown in FIG. 14; where not otherwise described,this embodiment of the invention includes features in common with thosediscussed above in connection with the '970 patent and the '817 and '743applications.

More specifically, according to one embodiment of this aspect of theinvention of the present continuation-in-part application, the range ofefficient use of the hybrid vehicle of the invention is furtherbroadened by providing a two-speed “range shifting” transmission, akinto those presently provided on SUVs and the like to allow shifting intoa “low range”, so that when the load is expected to be heavy forextended period of time, for example, when a heavy trailer is to betowed, the transmission can be operated to select the low range. Asindicated, such a transmission would normally only be operated once pertrip, and is accordingly not equivalent to a conventional multiple-speedtransmission which is operated to provide a sequence of effectiveoverall gear ratios each time the vehicle is accelerated, as suggestedin numerous prior art references dealing with hybrid vehicles. However,in another embodiment, the two-speed transmission thus provided could beoperated conventionally, i.e., shifted automatically duringacceleration, or in “kick-down” mode responsive to the operator's demandfor acceleration.

In one implementation of this aspect of the invention, as shown in FIG.14, a planetary gearbox 33 is disposed between the output shafts fromthe traction motor 25 and the combination of engine 40 and startingmotor 21. Gearbox 33 may be controlled directly by the operator, asconventional, or by the microprocessor 48, in response to an operatorcommand or responsive to sensing that the road load has exceeded somepredetermined value, e.g. 125% of MTO, for an extended time, e.g.several minutes, or conventionally, i.e., shifted under ordinaryacceleration. Typically the gearbox 33 will be locked, providing adirect drive, under ordinary circumstances; when a lower ratio isneeded, for example, when towing a heavy trailer, the gearbox 33 may becontrolled to yield a reduction of 0.5-0.8:1.

FIG. 14 also shows a second traction motor 222 driving a second set ofroad wheels 210 through a second differential 211. This is a convenientway of providing a “four-wheel drive” hybrid vehicle, which avoids thefore-and-aft driveshaft and third differential needed by conventionalfour-wheel drive vehicles. In this embodiment, road wheels 210 areconfigured as the steering wheels of the vehicle; accordingly halfshaftassemblies 212 incorporating universal joints are employed allowingwheels 210 to pivot, as illustrated. Traction motor 222 is connected tobattery bank (“BB” in FIGS. 14 and 15) via a further inverter/charger224, controlled by microprocessor 48 essentially similarly to tractionmotor 25. As noted above, a DC-to-DC converter 223 may be provided toallow the vehicle of the invention to be connected to vehicles havingconventional 12 volt electrical systems for emergency starting purposes,and to provide 12 VDC for operation of conventional accessories.

Provision of separate traction motors 222 and 25 with respect to thecorresponding pairs of road wheels 210 and 34 has several advantageswith respect to conventional vehicles; as noted above, the fore-and-aftdriveshaft and third differential normally required are eliminated,freeing substantial space normally required by these components.Further, “traction control”—that is, control of the amount of torquedirected to each pair of wheels responsive to the traction conditions,which is useful in driving in snow or mud, or on wet or icy pavement—isconveniently accomplished by the microprocessor, simply by monitoringthe wheels' response to given amounts of current and reducing thecurrent to spinning wheels.

As shown by FIG. 14, vehicles according to the invention provided withtwo traction motors and having a planetary gearbox 33 between onetraction motor and its corresponding road wheels may have a similargearbox 213 between the second traction motor 222 and its wheels;however, this second gearbox 213 is not expected to be commonlyrequired. Similarly, second traction motor 222 can be configured as ahigh-RPM unit, with its output shaft connected to the road wheelsthrough reduction gears 214. In this implementation startermotor/generator 21 is also shown connected to the road wheels through areduction device 34, illustrated as a chain drive; as indicated above,providing a mechanical reduction between the various motors 21, 25, and222 and the respective road wheels is desirable in order that the motorscan be selected and optimized to operate at higher speeds than engine40.

Another possibility not shown specifically by FIG. 14, but within thescope of the invention, is to provide a “torque converter” ofessentially conventional design, preferably fitted with a “lock-up”clutch, between the traction motor(s) and the corresponding wheels. Asis well known, torque converters are commonly employed as part ofautomatic transmissions for passenger cars; the torque convertermultiplies the input torque at low speeds. Such a torque converter wouldprovide increased acceleration from rest. However, a similar effect canbe obtained more simply by overdriving the traction motor(s) beyondtheir rated power for the first few seconds of acceleration.

Braking System

Numerous patents, including the '970 patent discussed above, recognizethat one advantage of hybrid vehicles is that by appropriate control ofelectric motor/generators connected to the road wheels, a substantialfraction of the energy lost by conventional vehicles to friction can berecovered through regenerative braking, that is, by converting thevehicle's kinetic energy to stored battery power by using torqueavailable at the road wheels to drive the motor(s) in generator mode,and storing the resulting electrical energy in the battery bank for uselater. It is commonly estimated that most of the energy expended inaccelerating the vehicle in city driving can be recovered in this way,since irrecoverable losses due to air resistance and rolling resistancecontribute relatively little to the vehicle's energy demands at lowspeeds; by comparison, less of the energy expended to drive the vehicleat highway speeds can thus be recovered, although regenerative brakingis nonetheless desirable.

More particularly, it is known to operate the motor/generator andcooperating inverter/charger electronics of hybrid vehicles so thatelectrical power is generated and stored in the battery bank when theoperator desires to slow the vehicle. Accordingly “regenerative braking”per se is known. It is generally also apparent to those of skill in theart that a conventional mechanical braking system must also be provided,both for safety in the event of a failure in the regenerative brakingsystem and to provide braking in the event the battery bank is fullycharged; that is, it is important to avoid overcharging the battery bankin order to maximize its useful life. See Boll U.S. Pat. No. 5,788,597and Frank U.S. Pat. No. 5,842,534. Similarly, mechanical braking is alsoneeded when regenerative braking is not possible, e.g., at a stop.However, the art known to the inventors does not address all theconcerns relevant to provision of a braking system of a hybrid vehicle,and to do so is another object of the present invention. See, e.g.,Mikami et al U.S. Pat. No. 5,839,533, which suggests employment ofengine braking (i.e., retardation of the vehicle using torque due tocompression of air in the engine, and friction therein) as well asregenerative braking. The choice between the two is apparently to bemade by the operator, at least in part responsive to the battery's stateof charge. This would be far too complex for general acceptance.

The disclosure of the Boll patent itself is directed to optimizing theuse of regenerative, engine, and mechanical braking. Boll alsorecognizes the desirability of maintaining a consistent brake pedal“feel” in the various brake modes.

German patent application DT 19 05 641 B2 to Strifler discloses acombined regenerative and mechanical braking system for an electricvehicle, wherein regenerative braking is effected upon the operator'sfirst operating a brake lever, and mechanical braking is furthereffected upon reaching the maximum regenerative braking effect. If thebattery cannot accept further charge, the mechanical braking istriggered relatively earlier, so that the operator experiencessubstantially the same pedal “feel” regardless whether regenerative ormechanical braking is being implemented.

The present invention also recognizes that providing proper brake “feel”to the operator is important to provision of a satisfactory vehicle, butdiffers substantially from the teachings of the art, and the Boll andStrifler references in particular, in the type of pedal feel preferred.

More particularly, it will be appreciated that typical vehiclemechanical brake systems provide a relatively linear relationshipbetween the force exerted on the brake pedal and the retarding forceexerted on the wheels by the brakes. It is essential that thisrelatively linear relationship be provided by the brake system of anyhybrid vehicle, so that the operator can smoothly and controllably brakethe vehicle as desired.

Providing a relatively linear relationship between the force exerted onthe brake pedal and the retarding force exerted on the tires by thebrakes is substantially straightforward in the case of conventionalmechanical braking systems. It is much more complicated in the case of abrake system incorporating regenerative braking as described above,since such a system must provide a linear relationship between the forceexerted on the brake pedal and the retarding force exerted on the tiresby the brakes and motor/generator(s) under all circumstances. Theproblem is particularly complicated during transitions from one brakingregime to another. For example, if regenerative braking is used tocommence deceleration but hydraulic braking must take over, e.g., if thebattery bank's state of charge becomes full during a long descent, or ifa leisurely stop suddenly becomes abrupt, the braking regime must changesmoothly and controllably. Regenerative braking is also not availablewhen the vehicle is moving very slowly or is at rest, and mechanicalbrakes must be available under these circumstances.

In addition to maintenance of the linear relationship, it is deemedpreferable by the present inventors that the operator be made aware by achange in the “feel” of the brake pedal that regenerative braking is notavailable, typically due to the battery bank's state of charge becomingfull. As noted, this is contrary to the teachings of the Boll patent andthe Strifler German application. More specifically, it is considereddesirable by the inventors that the brake pedal resist depression by theoperator to a degree proportional to the amount of regenerative brakingactually being effected at all times.

Finally, it will be appreciated that the engine manifold vacuum asconventionally used to produce “power braking”, i.e., servo assistance,is not available to a hybrid vehicle if the engine is not running; someother source of power for servo assistance is required in order thatbrake effort is not unacceptably high.

FIG. 15 shows schematically the principal components of a brake systemfor a hybrid vehicle that addresses the concerns above. Where commonreference numerals are employed, the components are common with thoseshown in other Figures, while components not important to understandingof the braking system are omitted for simplicity. Thus, FIG. 15 showstraction motors 222 and 25 connected directly to the respective roadwheels 210 and 34 respectively, omitting the other components discussedabove. (In vehicles where a single traction motor drives a single pairof wheels, the improvements described herein would be provided as tothese, while a four-wheel hydraulic braking system would also beprovided.) As also discussed above, motors 222 and 25 are connected tobattery bank 22 through respective inverter/chargers 224 and 27.Inverter/chargers 224 and 27 are controlled by microprocessor 48 tooperate so that the motors can draw power from battery bank 22 andimpart torque to the respective wheels to propel the vehicle in theappropriate modes of vehicle operation; during regenerative braking,inverter/chargers 224 and 27 are controlled so that the motors absorbtorque from the wheels, slowing the vehicle, and storing the power thusgenerated in the battery bank 22.

Control of the inverter/chargers and motors to absorb a desired amountof torque from the wheels in response to a braking command frommicroprocessor 48 is considered to be within the skill of the art. Thecommand itself may be determined by microprocessor 48 responsive to thedegree to which brake pedal 70 is depressed, as measured by apotentiometer or similar device, indicated at 71. However, according tothe invention, as above, a device is provided which varies the “feel” ofthe pedal (essentially its resistance to being depressed by the driver)responsive to the degree regenerative braking is in fact beingimplemented, thereby providing tactile feedback to the driver enablingsmooth deceleration and, when appropriate, also providing an indicationthat regenerative braking is not available.

In the implementation of the invention shown, controllable resistance tothe movement of brake pedal 70 is provided by connecting it to amicroprocessor-controlled pneumatic cylinder assembly 230. A piston 232fitting within a pneumatic cylinder 238 is driven by a connecting rod234 attached to pedal 70 by a clevis 236. As the pedal is depressed,moving from right to left in FIG. 15, i.e., from the position shown infull to that shown in dotted lines, piston 232 expels air from theinterior of cylinder 238 via vent 240. The rate at which air is expelledin response to any given pedal pressure is controlled by the spacing ofa needle valve 242 from a seat 244; the needle valve 242 is moved closerto its seat 244 to increase the resistance to airflow, or moved awayfrom seat 244 to reduce the resistance. The spacing is controlled bymicroprocessor 48 in order to vary the feel of the brake pedal 70; inthe implementation shown, the needle valve 242 is threaded into the bodyin which valve seat 244 is formed, and the spacing is controlled by themicroprocessor 48 by commands sent to a motor 248 rotating the needlevalve 242 through a pair of gears 250. A spring 252 may be provided toreturn the pedal to its initial position. Thus, for example, ifregenerative braking is not available, needle valve 242 is opened, sothat the cylinder provides little resistance to the pedal, effectivelyinforming the driver that only hydraulic braking is available. Whenregenerative braking is initiated, responsive to the microprocessor'sdetecting a signal from potentiometer 71, the needle valve is closedresponsive to the degree of braking provided, resisting motion of thepedal 70, and so that the pedal feel provided to the operator isresponsive to the degree of regenerative braking actually beingeffected. Obviously, numerous other arrangements to thus controllablyvary the feel of the brake pedal will occur to those of skill in theart.

The mechanical design of the hydraulic braking system of the hybridvehicle according to the invention is generally conventional, with twoprincipal exceptions as follows: First, as the engine is not alwaysrunning during movement of the hybrid vehicle, there is no consistentsource of manifold vacuum as conventionally employed to provide servoassistance to braking. Therefore, a motor 254 powered directly by thebattery bank BB is provided, and drives a vacuum pump 256, providingvacuum to a conventional servo booster 258, in turn operatingconventional wheel brakes 260. The same motor 254 can be used to powerother “ancillary” systems that in conventional vehicles are powered bythe engine, such as the power steering pump and the air conditioningcompressor. (The art does recognize that hybrid vehicles requiredifferent sources of power for ancillary devices, such as power steeringpumps or power brake pumps. See Heidl U.S. Pat. No. 5,249,637, at col.1, lines 7-45.) Second, in order that the initial movement of the brakepedal 70 activates only the regenerative braking process (in order toobtain the maximum benefit therefrom), a mechanism is provided so thatthe rod 262 actuating the piston within master cylinder 264 and thencethe wheel brakes 260 moves a distance X before the master cylinderitself is actuated. In the implementation shown, this mechanism simplyinvolves provision of a cross-pin 266 fixed to rod 262 and slidingwithin a slot 268 formed in the piston rod 270 of master cylinder 264;accordingly, the master cylinder piston(s) do not begin to move untilthe cross-pin 266 reaches the left end of slot 268. If the overall pedaltravel Y is six inches, the distance X defined by slot 268 may be suchas to allow pedal 70 to move freely through 1½ inches before thepiston(s) of the master cylinder 264 begins to move.

Thus, according to this aspect of the invention, potentiometer 71provides a signal to the microprocessor 48 when the brake pedal 70 isdepressed by the driver. The microprocessor 48 evaluates the batterybank state of charge (SOC) as indicated at 66; unless this is such thatfurther charging is undesirable, the inverter/chargers 224 and 27 areoperated such that motors 222 and 25 are operated as generators, so thattorque provided to the wheels by the road is converted into electricalpower, retarding the vehicle and charging the battery bank. The degreeof retardation thus provided depends on the degree to which pedal 70 isdepressed. The driver feels resistance to depressing the pedal from airresistance controlled by the opening of needle valve 242; microprocessor48 controls the opening of valve 242 so that the pedal feel correspondsto the degree of regenerative braking that is provided. In the eventregenerative braking is not available for some reason, perhaps becausethe battery bank is fully charged, because of some flaw in the chargingcircuits, or because the vehicle is stopped, valve 242 is opened, sothat the driver feels little resistance to initial pedal travel, untilthe hydraulic brake system is activated.

It will be apparent that other types of devices for controlling theresistance to pedal travel to correspond to the amount of regenerativebraking being provided, and thus to provide the desired linearrelationship between pedal resistance and vehicle retardation, could besubstituted for the pneumatic cylinder with microprocessor-controlledvent device shown. For example, a device controllably varying thefriction between the pedal pivot and its mounting structure could beprovided; a hydraulic system, similarly controlling the resistance toflow of a fluid through an orifice, might be provided; or a devicevarying the preload of a return spring might be provided. Otherequivalent devices for achieving the same goals will occur to those ofskill in the art.

HVAC System

The essential components of the heating, ventilation and airconditioning (HVAC) systems of conventional vehicles are a heater core,connected to the engine cooling system, an air conditioning systemincluding an evaporator, and a fan to blow air over the heater core andevaporator and into the passenger cabin. There are several issues to beaddressed in adapting the conventional automotive HVAC system to use ina hybrid vehicle. One is that conventionally the air conditioningcompressor is driven by the engine through an electrically-controlledclutch; in a hybrid vehicle this is unacceptable, as the engine is notrun constantly. Therefore the air conditioning compressor must bepowered differently. Similarly, again as the engine is not runconstantly, the heater core cannot be relied upon to heat the cabin.

The art does recognize that hybrid vehicles require different sources ofpower for ancillary devices, such as power steering pumps or power brakepumps. See Heidl U.S. Pat. No. 5,249,637, at col. 1, lines 7-45. Heidl'sdisclosure is to the effect that a motor/generator used to drive theancillaries during electric operation can be used as a generator whenthe vehicle is propelled by an internal combustion engine.

FIG. 16 shows the principal components of an HVAC system for a hybridvehicle according to the invention. The complex ducting that istypically provided to supply conditioned air throughout the vehiclecabin is represented by a single duct 300. A fan 302 forces air throughthe duct 300, and in succession past an evaporator 304, a heater core306, and an electric heater 308. The evaporator 304 is connected to anair conditioning compressor 310 driven by an electric motor 312 poweredfrom the battery bank, so that the air conditioning system can beoperated independent of the engine 40.

Motor 312 could be the same motor used to power other ancillaries, suchas the vacuum pump 256 (FIG. 15) used to provide servo assistance to thebrake system, or could be a separate motor dedicated to powering thecompressor 310. The latter may be preferred, as this would allowelimination of the clutch otherwise needed to permit operation of thecompressor only when needed; elimination of the clutch would also allowelimination of seals that are a source of leaks. Another advantage ofdriving the compressor from the battery bank according to the inventionis as follows. Conventionally, in order to be useful under allcircumstances, the compressor must be sized to provide full cooling withthe engine at idle. Such a compressor is very inefficient at higherspeeds; by decoupling the compressor from the vehicle drivetrainaccording to the invention, it can be designed to be driven by motor 312at a single optimally efficient speed. Cabin temperature can bethermostatically controlled by a throttling valve controlling the flowof refrigerant, or by turning motor 312 on and off as required. Theother components of the air conditioning system, including an expansionvalve 314 and a condenser 316, are shown schematically, and aregenerally conventional.

When the engine is running, it is efficient to employ waste heat fromthe engine cooling system to provide cabin heat, and accordingly anessentially conventional heater core 306 and control elements (notshown) are provided; heater core 306 is downstream of the evaporator 304with respect to the flow of air through duct 300, as conventional, sothat dehumidified air can be heated to provide efficient demisting.

In order to provide heat as may be required when the engine is notrunning, an electric heating element 308, essentially comprising a coilof Nichrome wire or the like, is provided, again downstream of theevaporator 304. Heating element 308 is provided with conventionalcontrols (not shown) and is powered directly from the battery bank 22,as indicated.

It will be appreciated that according to this aspect of the invention,suitably heated or cooled cabin air is thus available regardless of themode of operation of the vehicle, as needed in order that the hybridvehicle of the invention suffers no comfort or convenience drawback withrespect to conventional vehicles. Indeed, because ample electrical poweris available from the large battery bank of the hybrid vehicle, electricheater 308 can be designed to heat the cabin much more rapidly than doesthe coolant heat exchanging core of a conventional engine, thusproviding a convenience advantage. Similarly, conductors can be embeddedin the vehicle windows and windshield and powered by the battery bankfor improved electrically-operated de-misting and de-icing.

It will be appreciated that the hybrid vehicle and operational strategytherefor of the invention provide numerous advantages over the prior artdiscussed herein, and that further improvements and modificationsthereto are within the skill of the art. Accordingly, while a preferredembodiment of the invention has been disclosed, and various alternativesmentioned specifically, the invention is not to be limited thereby.

1. In a method of controlling an internal combustion engine of a hybridvehicle, said engine being operatively connected to drive wheels of saidvehicle through a clutch, said vehicle further comprising a tractionmotor operatively connected to drive wheels of said vehicle, astarter/generator motor operatively connected to said engine forstarting said engine and for providing electrical power in response totorque from said engine, a battery bank adapted to store electricalenergy to power said traction motor and to start said engine, at leastone inverter/charger adapted to cooperate with said traction motor andsaid starter/generator such that said traction motor can be operated toprovide torque to said road wheels responsive to electrical power fromsaid battery bank, or to provide electrical power to said battery bankresponsive to torque from said road wheels, and such that saidstarter/generator can be operated to provide torque to start saidengine, or to provide electrical power to said battery bank responsiveto torque provided by said engine, and a microprocessor adapted tocontrol operation of said engine, said traction motor, saidstarter/generator, and said at least one inverter/charger, so as tocontrol flow of torque and electrical power the rebetween in response tosensed parameters, the improvement comprising: establishing at leastfour vehicle operating modes, including: a mode I, wherein said engineis not operated and said vehicle is propelled by torque from saidtraction motor in response to electrical power drawn from said batterybank; a mode II, wherein said vehicle is propelled by torque from saidtraction motor in response to electrical power drawn from said batterybank, and said starter/generator is driven by torque provided by saidengine to provide electrical power to recharge said battery bank; a modeIII, wherein said vehicle is propelled by torque from said engine; amode IV, wherein said vehicle is propelled by torque from said engineand from said traction motor in response to electrical power drawn fromsaid battery bank; wherein said microprocessor controls operation ofsaid engine, said traction motor, said starter/generator, and said atleast one inverter/charger in response to the instantaneous torquedemands (RL) of said vehicle, and such that said engine is operated onlyin response to a load equal at least to a predetermined minimum value ofits maximum torque output.
 2. The method of claim 1, wherein saidstarter/generator is sized with respect to said engine such that saidstarter/generator is capable of being driven by said engine in said modeII while said engine produces at least about 30% of its maximum torqueoutput.
 3. The method of claim 2, wherein said battery bank is sizedsuch that the charging current supplied by said starter/generator inresponse to torque from said engine while producing at least about 30%of its maximum torque output is no more than about 50 amperes.
 4. Themethod of claim 1, wherein said microprocessor controls operation ofsaid vehicle such that said mode III is entered only when RL is at leastequal to a predetermined fraction of the engine's maximum torque output(MTO).
 5. The method of claim 4, wherein mode III is entered only whenRL is substantially equal to at least 30% of MTO.
 6. The method of claim5, wherein said vehicle is operated in mode III while 30%<RL<100% ofMTO.
 7. The method of claim 1, wherein mode IV is entered only whenRL>100% of MTO.
 8. The method of claim 1, wherein said vehicle furthercomprises a turbocharger adapted to be controlled by said microprocessorso as to increase the torque output of said engine from its maximumvalue while normally aspirated (MTO), and wherein a further vehicleoperating mode V is established, wherein said turbocharger is controlledto operate when RL is greater than MTO for more than a given period oftime T.
 9. The method of claim 8, wherein if said vehicle is in saidmode IV, with RL between 30 and 100% of MTO, and if RL then exceeds 100%of MTO, torque required in excess of 100% of MTO is initially providedby said traction motor, and if RL continues to exceed 100% of MTO formore than a given period of time T, said turbocharger is activated bysaid microprocessor such that said engine produces torque in excess of100% of MTO.
 10. A brake system for a hybrid vehicle, said vehiclecomprising a drive train including an internal combustion engineoperated to provide vehicle propulsive torque only during predeterminedmodes of operation of said vehicle and at least one traction motor andcorresponding inverter/charger adapted to provide vehicle propulsivetorque during predetermined modes of operation of said vehicle and toprovide electrical energy responsive to torque from wheels of saidvehicle during a regenerative braking mode of operation of said vehicle,a battery bank adapted to provide electrical energy to said motor asrequired and to accept charging energy from said motor when operated asa generator during said regenerative braking mode of operation of saidvehicle, and a microprocessor for controlling the mode of operation ofsaid vehicle, said brake system comprising: a brake pedal adapted to beoperated by a driver of said vehicle, a hydraulic brake system coupledto said brake pedal and comprising at least one master cylinder and anumber of wheel brakes operatively connected to said master cylinder forretarding said vehicle upon actuation of said pedal, a sensor forproviding a signal to said microprocessor responsive to motion of saidbrake pedal, a sensor for providing a signal to said microprocessorresponsive to the state of charge of said battery bank, a devicecontrollable by said microprocessor to vary the resistance to motion ofsaid pedal during braking responsive to the amount of regenerativebraking being provided, wherein said microprocessor controls the amountof regenerative braking provided upon motion of said pedal responsive tothe state of charge of said battery bank, and controls the resistance tomotion of said pedal during braking responsive to the amount ofregenerative braking being provided.
 11. The brake system of claim 10,wherein said device controllable by said microprocessor to vary theresistance to motion of said pedal during braking responsive to theamount of regenerative braking being provided comprises a pneumaticcylinder having a piston sliding therein, said piston being operated bysaid brake pedal, and comprising a vent passage including an orificecontrollable by said microprocessor to control the resistance to motionof said pedal.
 12. The brake system of claim 10, wherein said at leastone master cylinder is coupled to said brake pedal by an actuating rodarranged so that said pedal can be moved through a predetermineddistance before said master cylinder begins to apply pressure to saidwheel brakes.
 13. The brake system of claim 10, wherein said hydraulicbrake system comprises a servo actuator and a vacuum pump driven by amotor responsive to electrical power supplied from said battery bank.14. A heating, ventilation, and air conditioning (HVAC) system for ahybrid vehicle, said vehicle comprising a drive train including aninternal combustion engine run only during predetermined modes ofoperation of said vehicle and at least one traction motor adapted toprovide vehicle propulsive torque during predetermined modes ofoperation of said vehicle, a battery bank adapted to provide electricalenergy to said motor as required, said HVAC system comprising: a ducthaving a fan disposed therein for forcing air along said duct; anevaporator in said duct; an air conditioning compressor connected tosaid evaporator, and driven by an electric motor powered by said batterybank; a heater core in said duct and connected to a cooling system ofsaid engine; and an electrical heating element in said duct andconnected to said battery bank.
 15. The HVAC system of claim 14, whereinsaid evaporator is disposed in said duct upstream of said heater coreand said electrical heating element with respect to the direction of airflow through said duct.
 16. A method for determining the relative sizesof the internal combustion engine, starting/charging and tractionmotors, and battery bank of a hybrid vehicle comprising said components,said method comprising the steps of: a. selecting an internal combustionengine having sufficient torque to drive the vehicle without trailer atmedium to high speed along a moderate grade; b. sizing thestarting/charging motor to provide an engine load during batterycharging equal to at least approximately 30% of the engine's maximumtorque output; c. sizing the traction motor to provide adequate torqueat zero speed to overcome the maximum grade specified from rest, withthe starter motor assisting as needed; d. determining the maximum powerdrawn by the selected motor under full power conditions; e. calculatingthe battery voltage under load that will be required to provide thepower to be drawn by the motor(s) under full power conditions, and sothat the ratio of the battery voltage under load to the peak currentdrawn by the motor(s) is at least 2.5:1, and f. selecting the batterybank to provide the calculated voltage under peak load conditions.